SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section 213
 1. Introduction
 The Nördlinger Ries is a circular, flat depression of
 22-24 km diameter separating the Jurassic limestone
 plateaus of the Franconian and Swabian Alb in
 Southwest-Germany (Fig. 1). Its centre is located 110
 km NW of Munich, 70 km SSW of Nuremberg and
 110 km E of Stuttgart. The Ries basin was formed
 approximately 15 Million years ago (Gentner & Wag-
 ner, 1969; Staudacher et al., 1982) by an impact of a
 stony meteorite less than 1 km in size (Shoemaker &
 Chao, 1961; Stöffler, 1977).
 The Ries crater represents one of the best preserved
 and best investigated impact structures on Earth
 (Bayerisches Geologisches Landesamt 1969, 1974,
 1977; Hüttner & Schmidt-Kaler 1999; Stöffler &
 Ostertag 1983). It gained wide public attention (e.g.,
 Metz 1974, Steinert 1974, Lemcke 1981, Kavasch
 1985, Pösges & Schieber 1994; Schieber 2004) and
 served as training site for Apollo 17 astronauts in
 August 1970 (e.g., Margolin 2000).
 However, apart from its impact nature, the Ries basin
 offers a great opportunity to study fossil lacustrine
 microbialites. Such lacustrine deposits within impact
 structures are of increasing interest for understanding
 the origin and evolution of early life on Earth, and
 possibly other planets (Cockell & Lee 2002, Osinski
 et al. 2005, Cabrol et al. 2001). Therefore, the focus
 on this field trip is on microbial and algal build-ups,
 their facies context, and the discussion of microbial
 effects and lake water chemistry.
 2. The Ries impact
 Conclusive evidence for the formation of the Ries
 basin by a cosmic impact was drawn by Shoemaker
 & Chao (1961) and Chao & Littler (1963), who dis-
 covered the high-pressure polymorphs coesit and
 stishovit in the suevite, an impact-melt-bearing breccia
 (Hüttner 1969, 1977). More recently, diamonds and
 silicon carbide were detected within the suevite
 (Hough et al. 1995), as well as an ultradense rutile
 polymorph in shocked gneisses (El Goresy et al.
 2001). Today, the following cratering model is gene-
 rally accepted (Gall et al. 1975, Pohl & Gall 1977):
 A stony meteorite with high velocity penetrated
 through approximately 600 m Jurassic and Triassic
 sediments down into the crystalline basement rocks
 of the Moldanubicum. After the vaporization of the
 projectile and the excavation of a transient crater with
 a depth of 2 - 2.5 km, gravity caused collapse of the
 crater rim and uplift of the its floor. The final result
 was a hydrologically closed, complex impact structure
 almost 25 km in diameter, subdivided by a ring
 structure in a central crater and a marginal megablock
 zone (Reich & Horrix 1955, Chao 1977, Ernstson &
 Pohl 1977, Stöffler 1977). The ejecta blanket (mainly
 „Bunte Breccia“) as well as the crater floor was
 covered by a blanket of the
 impact-melt bearing
 crystalline breccia. Today,
 large parts of the „fallout
 suevite“ outside the crater are
 eroded (see Hüttner &
 Schmidt-Kaler 1999). As a
 consequence of the gravity
 modification described
 above, the later Ries crater
 lake had a high surface to
 depth ratio (Fig. 2), if
 compared to lakes in simple
 craters.
 Sediments of the Ries Crater Lake (Miocene, Southern Germany)
 GERNOT ARP
 Geowissenschaftliches Zentrum der Universität Göttingen, Goldschmidtstraße 3, 37077 Göttingen
 Fig. 1. Location map of the Ries impact crater with excursion route and stops.
 Field Trip F 2:
214 SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section
 3. The Miocene Ries crater lake
 3.1. Overview on the sedimentary succession
 Sediments of the Ries crater lake are preserved within
 the central basin (more than 300 m, mainly fine-
 grained clastics) and as erosional remnants (mainly
 carbonates and coarse-grained clastics) covering hills
 of the crystalline ring and slopes of the crater rim (Fig.
 2; Bayerisches Geologisches Landesamt 1969, Hütt-
 ner & Schmidt-Kaler 1999). Large parts of the lake
 history were documented by the research drilling
 Nördlingen 1973, which cored a lacustrine sequence
 of 314.3 m (Bayerisches Geologisches Landesamt
 1974, 1977). According to the comprehensive paper
 of Jankowski (1981), the lacustrine sequence can be
 divided into four sequences (Fig. 2):
 (1) The „basal sequence“ (58.3 m) consists of coarse-
 grained clastics of alluvial fans, grading into fine-
 grained carbonate-bearing mudstones of a playa and
 fossiliferous sediments of a permanent lake.
 (2) The following „laminite sequence“ (145 m)
 shows bituminous dolomitic laminites with abundant
 diatoms in its upper part. The laminites were deposited
 during eutrophic and often alkali-saline conditions of
 a permanent lake with euxinic bottom water.
 (3) Sediments of the „marl sequence“ (59 m) are
 formed by bedded, grey to greenish marls with bio-
 turbation and desiccation structures. Thus, the alter-
 nating saline and less saline lake lost its permanent
 stratification.
 (4) Sediments of the „clay sequence“ (52 m) comprise
 dark-grey, partly carbonaceous clays and thin lignite
 seams of an episodically non-saline, very shallow lake.
 The carbonate and clastic sediments of the Ries crater
 margins (i.e., the sediments visited during this
 excursion: Fig. 3) represent the last phase of the Ries
 lake, thus, are probably younger than the basinal
 succession described above (see discussion in Jan-
 kowski 1981; for contrary view see Wolff & Fücht-
 bauer 1976 and Schauderna 1983). Therefore, more
 than 100 m of lake basin sediments, which once filled
 up the complete crater basin, were removed from the
 central basin since the early Pleistocene (Wolf 1977).
 The Miocene Ries crater sediments now exposed at
 the surface can be subdivided into several facies units
 described below.
 3.2. Facies units
 Lacustrine clastics of the basin centre
 Description: Claystones and marlstones constitute by
 far the greatest portion of the Ries crater lake sedi-
 ments (Fig. 2). Permanent surface outcrops, however,
 do not exist.
 The rare temporary surface exposures by construction
 pits commonly show dark-grey to greenish-grey,
 laminated claystones with white-grey chalky calcite
 precipitates forming nodules or coatings on bedding
 planes (e.g., Wallerstein: Hollaus 1969; Wemding:
 Bolten et al. 1976; Oettingen: Peters 2003; Trendel:
 Knollmann 2003). Locally, cm- to dm-thick beds of
 laminated limestones are intercalated, consisting of
 micrite and microspar (e.g., Nittingen). Less abun-
 dant in the few surface outcrops are blue-grey mas-
 Fig. 2. Cross-section of the Ries crater basin showing major lacustrine facies units. Upper part shows a general section of the shallow basin,
 divided by a crystalline ring into a central crater and a marginal block zone. Lower part shows two detailed, exaggerated vertical scale sections
 of the marginal carbonate sediment cover with the location of spring mounds and various bioherms. From Pache et al. (2001), modified.
SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section 215
 sive clays and dark-grey to brownish, bituminous
 shales (e.g., road-cutting SE of Ehingen). Bituminous
 shales from drill cores have been shown to contain
 biomarkers of planktonic green algae (Botryococcan)
 as well as abundant organic sulfur compounds (Rull-
 kötter et al. 1990; Barakat & Rullkötter (1997).
 Claystones, oilshales as well as micrite beds locally
 show slumping structures (Bolten et al. 1976, Mertes
 1977, Arp 1995bc). Surprisingly, bedding planes with
 mud cracks have been observed in these basinal
 laminites (e.g., Wemding: Bolten et al. 1976).
 Most of the basinal sediments are poor in fossils.
 Hollaus (1969: 20) reported ostracods, few gas-
 tropods and fish scales from laminated clays exposed
 by a sewage plant construction pit at Wallerstein. One
 exceptional discovery was a fossiliferous bed with
 numerous fish skeletons and plant remains in laminated
 clay- and marlstones near Wemding (Bolten et al.
 1976).
 Interpretation: Laminated clay- and marlstones are
 considered as profundal sediments of a density-
 stratified saline lake. However, rare mud cracks in the
 laminites suggest that water depths were rather
 shallow and that even central parts of the lake excep-
 tionally suffered temporary desiccation. Bituminous
 intercalations reflect high primary production of
 planktonic green algae in the water column and anoxic
 bottom waters, whereas massive claystones represent
 intervals of water mixing and bottom water oxy-
 genation. Chalky carbonate precipitates probably
 resulted from recrystallization of instable primary
 carbonates and evaporites.
 Deltaic clastics of the basin margins
 Description: Coarse-siliciclastic deposits related to
 fluvial influx into the crater lake are
 known from several localities. They are
 widespread between Trendel, Ursheim
 and Megesheim in the NW sector of the
 Ries basin (Fig. 3), forming the „delta
 of Trendel“ (Bolten & Müller 1969).
 Further occurrences are known, e.g.,
 from the Ulrichsberg (stop 5; NE Ries;
 Groiss 1974), the vicinity of Ederheim
 (SW Ries; Nathan 1935) and south of
 Huisheim (SE Ries; Schröder & Dehm
 1950).
 The sediments comprise coarse
 conglomerates with cm-sized, poorly to
 well rounded pebbles within a greenish-
 grey sandy matrix. Pebbles are
 commonly 1 to 6 cm in diameter, rarely
 up to 20 cm. They consist of crystalline
 rocks or Upper Jurassic limestones,
 rarely of Middle Jurassic sandstones or kaolinized
 impact glass. The conglomerate beds form lenticular
 beds with a poorly visible cross-stratification (Groiss
 1974). Conglomerates and sandstones locally inter-
 finger with greenish-grey clayey sandstones and light-
 grey sandy dolomite beds with root traces, ostracods
 and Hydrobia trochulus.
 In the Trendel area, yellow-brownish friable sand-
 stones and ooid-bearing sandstones are widespread
 (Weber 1941, Bolten 1977). Some of them show a
 clear cross-stratification and patchy cementation. The
 latter causes a nodular weathering of the sandstones.
 Large-scale foreset beds in friable sandstones are
 exposed in the Megesheim sand pit (stop 9; Chao et
 al. 1987).
 Interpretation: Conglomerates and sandstones are
 largely deltaic deposits of fluvial inlets into the Ries
 lake, as already notified by v. Gümbel (1889). Their
 composition reflects the local availability of pre-
 Miocene rock types, such as crystalline rocks at the
 Ulrichsberg and Upper Jurassic limestones in the
 Ederheim area.
 Calcitic spring mounds
 Description: Spring mound carbonates, also known
 as „travertines“ in the Ries basin, form localized
 mounds and pinnacles with steep sides of meter to
 several decametre in size (Fig. 2 - 4). Large mounds,
 such as the Wallerstein (stop 7) and Alerheim castle
 rocks (stop 8), are commonly located on basement
 blocks of the crystalline ring structure (Wolff &
 Füchtbauer 1976; Bolten 1977; Bolten & Gall 1978;
 Pache et al. 2001). They are up to 30 m high and
 exhumed from their surrounding lake clays by
 Pleistocene erosion (Bolten 1977). Further spring
 Fig. 4. Reconstruction of the northern Ries lake margin with the
 distribution of major facies zones. From  Arp (1995b).
216 SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section
 Fig. 5. Spring mound carbonates.
 A. Upper Jurassic limestone clasts enclosed in basal spring mound carbonates. Main street, Trendel (Knollmann 2003).
 B. Field view of the northern, basal part of the Wallerstein castle rock showing barrel-shaped sickle-cell limestone cones veneered by thrombolite
 crusts. The core parts of these structures have largely been removed by quarrying. From Pache et al. (2001).
 C. Stromatolitic column of sickle-cell limestone at the stairway to the Wallerstein summit.
 D. Polished slab of sickle-cell limestone. N of Staudigberg.
 E. Pupal cases of flies preserved by passive incrustation at the margin of a spring mound. Leihbug N of Ehingen. SEM micrograph. From Arp
 (1995b).
 F. Negative cast of a dragonfly larva at a sinter-veneered fissure wall of a spring mound. Belzheim.
 mounds formed on top of brecciated Upper Jurassic
 limestones (Fig. 5A) of the marginal megablock zone
 (e.g., Goldberg; Wolff & Füchtbauer 1976; Bolten
 1977), and some few even formed at front parts of
 deltaic clastics (Weber 1941: p. 130, Abb. 10). Facies
 mapping revealed that spring mounds are much more
 widespread that previously through (Fig. 3).
 The „travertines“ consist of porous calcitic limestones
 with only traces of aragonite. In contrasts to the algal
SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section 217
 bioherms, microcrystalline
 dolomite and limpid dolomite
 cements are absent. Central
 parts of the mounds and
 pinnacles (Fig. 5B) are
 composed of so-called sickle-
 cell limestones (Fig. 5CD;
 „Sichelzellenkalke“; Reis
 1926). These are highly
 porous limestones consisting
 of 100-200 µm thick
 microcrystalline calcite sheets
 spanning over mm- to cm-
 sized lenticular to sickle-
 shaped voids. Bubble-like
 structures and networks of
 micrite threads occur as well
 (Pache et al. 2001). Laminated
 and dendroid sinter cements
 and curtains of thin stalactites,
 which veneer dissolution voids
 and fissures, too, stabilize this
 initial framework.
 Marginal mound parts show thrombolites, planar non-
 skeletal stromatolites, and rare green-algal frame-
 stones. First lacustrine ostracods and Hydrobia shells
 occur in these marginal mound carbonates, but most
 characteristic are elongated faecal pellets and insect
 larval tubes. However, sheet cracks and polygonal
 mud cracks are common in intercalated non-skeletal
 stromatolites.
 Well-preserved vertebrate remains, such as avian
 skulls, feather imprints, eggshells and diverse
 mammals, have been reported from the Ries spring
 mounds (Deffner & Fraas 1877, Bolten & Müller
 1969, Heizmann & Fahlbusch 1983, Kohring & Sachs
 1997, Mayr & Göhlich 2004). However, sickle-cell
 limestones, thrombolites and stromatolites are
 disappointingly poor in fossils. A closer look to the
 fossil locations and associated freshwater gastropods
 suggests that these vertebrate remains are restricted
 to fissure fillings (e.g., Kohring & Sachs 1997) or
 mound tops largely belonging to the final freshwater
 deposits of the Ries lake. Only accumulations of pupal
 cases of flies (Fig. 5E; Seemann 1935, Arp 1995bc)
 and larvae of dragonflies (Bolten 1977: 86; Fig. 5F)
 may coincide in time with the major mound formation.
 Interpretation: Spring mounds of the Ries are similar
 to that of present-day soda lakes such as Mono Lake
 (Russel 1889, Scholl & Taft 1964), Lake Van (Kempe
 et al. 1991) and Lake Nuoertu (Arp et al. 1998). The-
 se form at sublacustrine springs by mixing of Ca2+-
 supplying groundwater with bicarbonate-rich alkaline
 lake water (Fig. 6). As a result, cyanobacterial biofilms
 Fig. 6. Model of spring mound formation in the Ries soda lake. Periods of subaquatic growth and
 prolonged exposure alternated. After initial impregnation of microbial mats within the mixing
 zone, sickle-cell fabrics form by bacterial degradation and shrinkage of the mats (Arp 1995bc, Arp
 et al. 1998).
 flourishing at these spring sites become loosely CaCO3impregnated due to the extremely high supersaturation
 of mixing zone waters. Compared to the mixing pro-
 cess, photosynthetic activity is of minor importance
 in causing CaCO3 precipitation (Arp et al. 1999). How-ever, the characteristic sickle-cell fabric results from
 exopolymer degradation by heterotrophic bacteria,
 mucus shrinkage and secondary Ca2+ release from the
 exopolymers (Arp et al. 1998). This process only
 promotes calcification to form lithified microbialites
 if the dissolved inorganic carbon of the lake water
 buffers the simultaneously released CO2 (Arp et al.2003). Consequently, sickle-cell fabrics are consi-
 dered to be indicative of highly alkaline, high-DIC
 environments, i.e., soda lakes.
 Spring mounds growth in the Ries lake basin, how-
 ever, was affected by lake level fluctuations and
 recurrent subaerial exposure and karstification during
 their formation, as indicate by well-developed, multi-
 phasic speleothems (Fig. 6). To date, no evidence has
 been found that sublacustrine spring waters of the Ries
 lake basin have been thermal, contrasting with the
 assumptions of early investigators (v. Gümbel 1870,
 Klähn 1926, Seemann 1935, 1941). Consequently, the
 spring mound carbonates are no real travertines, but
 cool-water tufa deposits (cf. Riding 1991).
 Dolomitic algal bioherms
 Description: Highly porous algal bioherms are
 widespread at marginal hills of the Ries basin. They
 are composed of green algal dolomites, cyanobacterial
 stromatolites and thrombolites (Reis 1926, Wolff &
218 SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section
 Fig. 3. Facies map of lacustrine and deltaic sediments of the northern margin of the Ries basin. Compiled from Arp (1995c), Pache (2000), Peters
 (2003) and Knollmann (2003).
SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section 219
 Fig. 12. Facies map of a dolomitic green algal bioherm and associated carbonate sands cross-sectioned at the Hainsfarth quarry at the Büschel-
 berg (Arp 1995bc). Note that black lines indicate laminated sinter veneers, i.e., short-term discontinuities in bioherm growth.
220 SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section
 Fig. 7. Algal bioherm carbonates.
 A. Cladophorites-framestone of a Hainsfarth-type algal bioherm, Ehingen-Lindenstraße.
 B. Cladophorites-bafflestone of a Hainsfarth-type algal bioherm, NW of Maihingen.
 C. Accumulation of charophyte stem fragments in bioherm-associated carbonate sands with Hydrobia trochulus and Strandesia risgoviensis.
 Abandoned quarry NNW of Maihingen.
 D. Skeletal stromatolite of a Hainsfarth-type algal bioherm, N of Utzwingen. From Pache (2000).
 E. Non-skeletal stromatolite, road-cutting Ehingen-Belzheim.
 F. In situ brecciated stromatolite of a Staudigberg-type algal bioherm. Ehingen Eichenstraße.
 Füchtbauer 1976, Bolten 1977, Riding 1979, Arp
 1995bc), which are commonly veneered by rigid,
 laminated sinter crusts to form nodules, cones and
 compound cones (stop 1; Riding 1979). The total
 bioherm succession extends over an elevation dif-
 ference of 80 m, but the thickness of carbonate
 deposits at one place rarely exceeds 10 m.
 The major bioherm constituent is a fossil green alga
 closely resembling the modern Cladophora (Reis
 1926, Wolff & Füchtbauer 1976, Riding 1979. The
 fossil, termed Cladophorites incrustatus (Ludwig)
 Reis is composed of branching, erect tubes of 50-140
 µm inner diameter and dolomicritic walls of 5-15 µm
 thickness. Pure Cladophorites-framestones (Fig. 7A)
SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section 221
 exhibit a rhythmic
 growth pattern due to an
 alternation of erect tubes
 and branched, more
 densely arranged and
 tangled tubes, while high
 particle influx results in
 irregular Cladophorites-
 bafflestones (Fig. 7B).
 By contrast,
 charophytes are very
 rare in the algal
 bioherms. They are not
 involved in bioherm
 growth and occur as
 poorly calcified,
 allochthonous stem
 fragments only. Accu-
 mulations of charophyte
 stem fragments (Fig.
 7C), however, have been
 observed in bioherm-
 associated carbonate
 grainstones in the Mai-
 hingen-Markoffingen
 area, which is charac-
 terized by co-occurring
 deltaic conglomerates.
 C y a n o b a c t e r i a l
 microfossils are much
 more rare and restricted
 to few lithostratigraphic
 horizons exhibiting skele-
 tal stromatolites (Fig.
 7D). Erect filament tra-
 ces and microcrystalline
 tubes of 8-12 µm internal
 diameter may result from
 former Oscillatoria-type
 cyanobacteria, whereas
 1.5- to 2-µm-thin,
 undulating filaments as-
 sociated with them may
 be either cyanobacteria
 or Chloroflexus-type bacteria (Arp 1995bc). Brush-
 like arranged filament traces of 10-15 µm diameter
 without defined tube wall might result from
 Phormidium-type cyanobacteria. Coccoid micro-
 fossils of possible cyanobacteria, 10-15 µm diameter,
 have only been detected in one blackened stromatolite
 clast, redeposited in the palustrine limestones (Peters
 2003). By contrast, hollow spheres of 120-130 µm
 diameter, locally enriched in depressions between
 stromatolite domes, are arthropod eggs (Arp 1995bc),
 not coccoid green algae („Chlorellopsis“).
 Fig. 8. Electron microprobe analysis of Cladophorites-tubes, limpid dolomite cement and late meteoric
 Microcrystalline dolomite components of the Ries lake
 are strikingly rich in Sr (Wolff & Füchtbauer 1976).
 Apart from that, a limpid dolomite cement consisting
 of 5-15 µm sized idiomorphic rhombs is present in
 all of the bioherms. It is less rich in Sr, but shows
 strikingly high Na concentrations (Fig. 8).
 At some segments of the northern rim, the bioherms
 form an almost continuous „reef belt“ (Fig. 3, 4),
 while their distribution is more patchy in southern
 parts of the basin. Numerous small quarries expose
 these inverted-cone-shaped algal reefs up to a height
222 SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section
 Fig. 9. Cycle scheme of Hainsfarth-type algal bioherms. Lake level fluctuations due to short-term climatic changes are shown simplified by
 a sinus-like graph. Complete sequences are 1.5-2.5 m thick. From Arp (1995b).
 of 7 m. Three types of algal bioherms can be distin-
 guished on basis of their facies association:
 Adlersberg-type bioherms: These are the oldest
 known algal bioherms of the Ries basin, occurring as
 first, patchy bioherm areas covering small hills (stop
 6). The type locality Adlersberg quarry shows that the
 bioherms are composed of green algal dolomite cones
 and nodules (Cladophorites-framestones and -baffle-
 stones) veneered by laminated sinter crusts (Riding
 1979). However, the characteristic constituents of
 Adlersberg-type bioherms are intercalations of planar
 non-skeletal stromatolites (Fig. 7E), which indicate
 lake-level lowstands. The bioherm carbonates are
 associated with bedded skeletal pack- and grain-
 stones, largely composed of the ostracod Strandesia
 risgoviensis (Sieber) (see Janz 1995) and the meso-
 gastropod Hydrobia trochulus Sandberger. Pockets
 within the bioherms locally contain palustrine lime-
 stones with accumulations of the land snail Cepaea
 sylvestrina (Schlotheim). Adlersberg-type bioherms
 show 1 to 3 m thick sedimentary cycles, represented
 by an alternation of nodular to conical green algal
 dolomites and planar non-skeletal stromatolites (Riding
 1979, Jankowski 1981).
 Hainsfarth-type bioherms: This type of bioherms is
 younger than the Adlersberg-type bioherms and
 represents the maximum distribution of algal bioherms
 in the Ries basin (stop 1). At the northern margin of
 the Ries basin, Hainsfarth-type bioherms form an
 almost continuous bioherm belt between Maihingen
 and the „delta of Trendel“ (Fig. 3). Similar to Adlers-
 berg bioherms, Hainsfarth-type bioherms are mainly
 composed of green algal dolomite cones and nodules
 (Cladophorites-framestones and -bafflestones)
 veneered by laminated sinter crusts (Riding 1979; Arp
 1995bc; Fig. 7AB). Again, the bioherms are associated
 with bedded pack- and grainstones composed of
 ostracods (Standesia risgoviensis), mesogastropods
 (Hydrobia trochulus), ooids, peloids and intraclasts.
 Likewise, pockets of palustrine limestones with ab-
 undant Cepaea sylvestrina are common. Exceptional
 findings are thin-walled foraminifera, one specimen
 of the freshwater gastropod Brotia escheri, and a
 dolomitized millipede (Arp 1995abc).
 In contrast to Adlersberg-type bioherms, planar non-
 skeletal stromatolites do not occur any more. Instead,
 skeletal stromatolites (Fig. 7D) are developed at the
 top of bioherm cycles, just below discontinuities.
 These stromatolites are increasingly abundant to the
 top of the Hainsfarth-type bioherm succession. Here,
 skeletal stromatolites and stromatolite-incrusted wood
 fragments form a lithostratigraphic marker bed.
 Hainsfarth-type bioherms show a clear cyclic facies
 development (Fig. 9): Palustrine limestones with
 Cepaea sylvestrina are the first deposits of a cycle,
 commonly preserved in pockets of the preceding
 bioherm cycle. Bioherm growth starts with nodular
 Cladophorites-bafflestones, followed by cones of
 rhythmically grown Cladophorites-framestones, and
 further nodular Cladophorites-bafflestones. Cycle
 tops are characterized by skeletal stromatolites, sub-
 sequent erosion and pocket formation.
 Staudigberg-type bioherms: These are the youngest
 known algal bioherms of the Ries basin, occurring as
 relictic deposits at the northern basin margin near
 Ehingen (e.g., Staudigberg) and Oettingen (e.g.,
SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section 223
 Affenberg). Previously, these bioherms have been
 described as intraclastic carbonates forming an
 „aragonitic top horizon“ (Wolff & Füchtbauer 1976).
 However, temporary outcrops in Ehingen revealed that
 major parts of the lithologic unit are microbialite
 bioherms brecciated in situ by pedogenesis (Arp
 1995bc). Most striking are white, aragonite-bearing,
 brecciated stromatolites (Fig. 7F) within a peloidal,
 greenish-grey to brownish calcite matrix. Limpid
 dolomite cements are still present. In addition, grey,
 aragonite-bearing thrombolites are a major component
 of the 0.5 to 1 m thick bioherm cycles. In contrast to
 previous bioherms, green algal dolomites (Clado-
 phorites-baffle/framestones) do not occur any more.
 Hydrobia trochulus and ostracods are rare.
 Interpretation: Dolomitic algal bioherms of the Ries
 lake occupy large parts of the former lake shore. In
 contrast to spring mounds, there is to date no evidence
 of groundwater influx that possibly supported cal-
 cification in the bioherms. The cyclic development of
 the bioherms apparently reflects evaporation-driven
 lake level fluctuations due to short-term climatic
 fluctuations, because the Ries lake had no outlet at
 that time.
 Sinter-veneered Cladophorites-bafflestones probably
 formed by high-Mg calcitic impregnation of biofilms
 upon green algal filaments and baffling of
 allochthonous particles between the resulting tubes
 within the wave-exposed eulittoral. Primary aragonite
 precipitates were present, too, but were of minor
 importance. The laminated sinter crusts, characterized
 by pendent morphologies, are interpreted to be of
 lacustrine-vadose origin. Prolonged eu- to supralitt-
 oral conditions are reflected by non-skeletal
 stromatolites and skeletal stromatolites. On the other
 hand, intervals of lake level highstand and shallow
 sublittoral conditions allowed undisturbed, rhythmic
 growth of Cladophorites-framestones. These Clado-
 phorites growth rhythms are considered to reflect
 seasonal rhythms and suggest average annual accu-
 mulations of 4 mm for framestones. Assuming that
 Cladophorites-bafflestones grew only half as fast as
 the framestones because of seasonal exposure, a 2
 m thick bioherm composed of frame- and
 bafflestones may have grown within only 800 years.
 The bioherm-affecting short-term climatic cycles may
 therefore represent time intervals of several hundred
 to a few thousands of years. Conversely, much longer
 time periods may be represented by subaerial exposure
 and non-deposition. Dolomitization including dolomite
 cement precipitation was clearly phreatic and has been
 related to evaporative lake level fluctuation (Wolff &
 Füchtbauer 1976, Arp 1995bc). However, the exact
 mechanism (mixing zone dolomitization, seepage
 reflux, microbial) remains to be elucidated.
 Dolomitic carbonate sands
 Description: Coarse bedded pack- and grainstones
 lithified by limpid dolomite, fibrous calcite and blocky
 calcite cements are commonly associated with algal
 bioherms. However, where bioherm are patchy or
 absent (e.g., Herblingen-Hochaltingen area), these
 carbonate sands constitute a separate facies unit (Fig.
 3). This is also true for the transition to final freshwater
 deposits of the Ries lake, where algal bioherms are
 completely absent (e.g., Loher Kopf plateau and south
 of Breitenlohe; Fig. 2, 3).
 Most common facies types are ooid-grainstones,
 peloid-packstones with root traces (Fig. 10A),
 intraclast- and oncoclast- rudstones (Fig. 10B). How-
 ever, ostracod-pack/grainstones and Hydrobia-pack/
 grainstones dominate at lithostratigraphic levels
 equivalent to Hainsfarth-type bioherms. Cepaea shells
 are commonly broken into bioclasts and bored by
 endolithic algae. Locally, carbonate sands show a
 clear cross-stratification (e.g., Alerheim, Arp & Wies-
 heu 1997). In early lithostratigraphic levels, carbonate
 sands are associated with non-skeletal stromatolites
 that form the transition to spring mounds (e.g., road
 cutting Ehingen-Belzheim; Arp 1995bc). Later litho-
 stratigraphic levels show pedogenically modified pack-
 and wackestones, i.e. palustrine limestones, instead.
 Similar to algal bioherms, dolomitic carbonate sands
 show a cyclic succession. Pack- and grainstones
 alternate with erosional discontinuities or platy dolo-
 mite mudstones with polygonal mud cracks (Fig.
 10C). The latter appear to be restricted to lithostrati-
 graphic levels equivalent to Adlersberg-type bioherms.
 Interpretation: Dolomitic carbonate sands are the
 dominant lake shore deposits, between algal bioherms
 and where algal bioherms are absent (e.g., transition
 to final lacustrine freshwater deposits). While peloid-
 packstones with well preserved root traces (Fig. 10A)
 characterize shallow sublittoral conditions, skeletal
 pack/grainstones as well as intraclast- and oncoclast-
 rudstones are considered as deposits of a wave-
 exposed eulittoral. Non-skeletal stromatolites and
 palustrine limestones finally represent supralittoral
 plains. Climatic fluctuations causing prolonged lake-
 level lowstands and playa conditions are indicated by
 dolomite mudstones with mud cracks (Arp 1995bc,
 Arp & Wiesheu 1997).
 Palustrine limestones
 Poorly bedded limestones occur at the backside of the
 dolomitic algal bioherm belt, especially at the northern
 rim of the Ries basin (stop 2; Fig. 3-5). They
 interfinger with the bioherms, forming pocket and
 fissures fillings within them. Locally, greenish-grey
 marl intercalations have been observed, too. Palustrine
224 SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section
 Fig. 10. Dolomitic carbonate sands, palustrine limestones and final lacustrine freshwater deposits.
 A. Shallow sublittoral peloid-packstone with root traces. Road-cutting Ehingen-Belzheim.
 B. Intraclast-oncoclast-rudstone of the wave-exposed eulittoral. Road-cutting Ehingen-Belzheim.
 C. Platy dolomicrite with polygonal mudcracks of playa intervals. Alerheim village. From Arp & Wiesheu (1997).
 D. Palustrine limestone with complex desiccation cracks and abundant intraclasts. Abandoned quarry W of Loher Kopf. From Peters (2003).
 E. Polished slab of black-pebble-conglomerate. Blackened and unaltered bioherm clasts are embedded in a pedogenic matrix with
 caliche-like crusts. Höllbug NW of Ehingen.
 F. Bioturbated ostracod-wackstone rich in the freshwater gastropod Gyraulus kleini. Final lacustrine freshwater deposits WSW of Breitenlohe.
 limestones are composed of calcitic mud- and wacke-
 stones that show complex desiccation cracks (Fig.
 10D), dissolution-enlarged root traces, animal
 burrows and in situ formed grains (nodules, pedoge-
 nic ooids, pisoids). Complex desiccation cracks
 include skew planes, curved planes and craze planes
 (see Freytet & Plaziat 1982). Some of these crack
 systems show multiple sinter cement generations and
 an inverse gradation of intraclasts formed in situ (Fig.
 10D). The land snail Cepaea sylvestrina (Schlotheim)
 is very common in palustrine limestones. In addition
 to that, other pulmonate gastropods occur as well,
 such as Discus (Discus) costatus (Gottschick) and
 Granaria antiqua noerdlingensis (Klein) (Bolten 1977,
 Arp 1995bc).
SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section 225
 A peculiar facies type within the palustrine facies unit
 are black pebble rudstones (Fig. 10E), previously
 described as „algal nodule limestones“ (Bolten 1977;
 see also Andres 1952). These are conglomeratic lime-
 stones with blackened bioherm clasts up to 12 cm
 diameter (skeletal stromatolites, Cladophorites-baffle/
 framestones) embedded in pedogenic peloidal micrite.
 Pebbles show multiple incrustations by brownish,
 indistinctly laminated crusts of pedogenic origin (Ca-
 liche). These crusts locally contain the microproble-
 maticum Limnocodium, which is a variety of Micro-
 codium known from paleosols (Klappa 1978, Freytet
 & Plaziat 1982). Black pebble rudstones were found
 up to 300 m behind the bioherm belt and show a
 patchy distribution. They probably form dm-thick
 intercalated beds between palustrine limestones.
 Interpretation: Palustrine limestones represent a
 calcareous swamp or floodplain behind the algal
 bioherm belt (Arp 1995bc). Dissolution-enlarged root
 traces, complex desiccation cracks, in situ formation
 of grains and vadose cements point to a common
 alternation of flooding, swamp pedogenesis and
 subaerial exposure (see Freytet & Plaziat 1982). Black
 pebble rudstones are interpreted as deposits of
 episodic storm and flooding events that caused
 transport of bioherm clasts to elevated, exposed areas
 of the floodplain (Arp 1995bc). Here, prolonged
 subaerial exposure and paleosol formation caused
 blackening and caliche formation.
 Final lacustrine freshwater deposits
 Description: Only relictic deposits of this latest period
 of the Ries crater lake history are preserved. These
 are patchy remnant occurrences at high topographic
 levels at the northern and northeastern basin margin
 (Bolten 1977, Arp 1995bc; Fig. 2, 3). Furthermore, a
 number of marly fissure and pocket fillings of the
 Goldberg spring mounds belong to these youngest
 Ries lake sediments (Bolten 1977).
 Near Breitenlohe, final freshwater deposits comprise
 greenish-grey marls and clays with intercalated light-
 grey limestones bed (stop 3). These are bioturbated
 ostracod wackestones with root traces, freshwater
 gastropods, and poorly calcified charophyte remains.
 Within marls and marly limestones, gastropods still
 show their primary, aragonitic mineralogy (Fig. 10F).
 Most abundant freshwater gastropods are Gyraulus
 kleini (Gottschick & Wenz), Radix socialis dilatata
 (Noulet) and Planorbarius cornu mantelli (Dunker).
 It is noteworthy that Gyraulus kleini (Fig. 10F),
 which shows an intra-lacustrine speciation in the con-
 temporaneous Steinheim crater lake (Hilgendorf 1866,
 Mensink 1984, Gorthner 1992), does not show any
 evolutionary changes in the Ries crater lake.
 Other localities further east (i.e., Loher Kopf, nor-
 theast of Kreuzhof, top beds of the „delta of Tren-
 del“ near Ursheim) comprise light-grey to yellow-
 brownish quartz-rich oolites and calcareous
 sandstones. The deposits show gastropod faunas with
 Gyraulus kleini (Gottschick & Wenz), Radix socialis
 dilatata (Noulet), Planorbarius cornu mantelli
 (Dunker), Melanopsis kleini kleini Kurr, Theodoxus
 crenulatus crenulatus (Klein), Valvata radiatula
 radiatula Sandberger, and few unionid bivalves
 (Bolten 1977). However, temporary increased salinities
 are indicated by co-occurrence of the mesogastropod
 Hydrobia trochulus Sandberger at some the localities
 (Bolten 1977).
 In contrast to earlier deposits of the Ries crater lake,
 sediments of the final freshwater stage commonly
 show vertebrate remains (Bolten 1977). Also, fissure
 and pocket fillings at the top of spring mounds appear
 to belong to this late freshwater stage of the Ries
 crater lake. These deposits revealed remains of
 cyprinid fish, salamanders, tortoises, lizards, birds
 (among them ducks, cranes, parrots, pelicans and
 flamingo-related birds), and mammals (among them
 hedgehogs, rodents, hares, bats, and even-toed
 hoofed mammals; Deffner & Fraas 1877, Seemann
 1941, Bolten & Müller 1969, Ballmann 1979, 1983,
 Heizmann & Fahlbusch 1983, Ziegler 1983, Rachl
 1983, Heizmann & Hesse 1995) associated with
 freshwater gastropods (Bolten 1977).
 Interpretation: The marls, limestones and calcareous
 sandstones described above are considered as shallow
 sublittoral to low-energy eulittoral deposits of a final
 freshwater stage of the Ries crater lake. This final
 freshening period may have been caused by climatic
 change (increase in humidity, decrease in temperatu-
 res) and/or formation of an outlet (Wolff & Fücht-
 bauer 1976, Bolten 1977, Arp 1995bc).
 3.3. Palaeoenvironmental interpretation and present-
 day analogues
 Long species lists of fossils described from the Ries
 lake deposits, especially those from spring mound
 localities, suggest a generally hostile freshwater
 environment for the crater lake. However, large parts
 of the siliciclastic succession in the central basin
 recovered by the research drilling Nördlingen 1973
 show rather species-poor lacustrine fossil assem-
 blages (Dehm et al. 1977). Only early parts (e.g., at
 262 m core depth) and intervals near the top of the
 drilled sequence (less than 52 m core depth) provide
 evidence of reduced salinities and temporary
 freshwater conditions (Füchtbauer et al. 1977, Dehm
 et al. 1977, Jankowski 1977, 1981). The laminite
226 SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section
 Fig. 11. Computed effect of carbon assimilation on the CaCO3 supersaturation of natural waters as a
 function of carbonate alkalinity for present-day atmospheric pCO2 (Arp et al. 2001). The y-axis indicates
 the resulting change in the supersaturation index (SICc = log (IAP/Kso) denoted as DSICc. High-alkalinity
 waters (i.e., soda lakes) show only minor changes in CaCO3 supersaturation upon carbon removal, whereas
 low-alkalinity waters (i.e., hardwaters) are strongly affected.
 sequence, characterized by oil shales and autigenic
 minerals known to form under highly-alkaline con-
 ditions (analcime and clinoptilolite), apparently was
 deposited in a stratified soda lake (Jankowski 1981).
 Also, covariation of stable oxygen and carbon isoto-
 pes provide evidence for a hydrologically closed basin
 (Rothe & Hoefs 1977).
 Marginal carbonates of the Ries basin, considered by
 Jankowski (1981: 197) to be younger than the central
 lake deposits recovered by the drillings (e.g., Nörd-
 lingen 1973, Deiningen 1 and 2), are characterized by
 a strikingly species-poor but individual-rich fossil
 association. This is especially true for Adlersberg- and
 Hainsfarth-type bioherms and associated carbonate
 sands. The dominance of one ostracod species
 (Strandesia risgoviensis) and one mesogastropod spe-
 cies (Hydrobia trochulus) and the lack of freshwater
 molluscs indicate saline, probably brackish conditions.
 Increased alkalinities are indicated by sickle-cell lime-
 stones of the spring mounds. Today, such fabrics only
 form at sublacustrine spring sites of highly-alkaline
 salt lakes, i.e. soda lakes such as Lake Van in Anatolia
 (Kempe et al. 1991), Mono Lake in California (Russel
 1889, Scholl & Taft 1964) and Lake Nuoertu in the
 Inner Mongolia (Arp et al. 1998). Mass accumulations
 of pupal cases of flies preserved at the Ries spring
 mounds (Seemann 1935, Arp 1995bc) are also well
 known from various present-day halite and soda lakes.
 Lower parts of the marginal carbonates, i.e., Adlers-
 berg-type bioherms and stratigraphic equivalents, even
 show intercalations of playa sediments and traces of
 former evaporite minerals.
 Besides of cyanobacteria, algal bioherms are
 dominated by only one green algal genus related to the
 extant Cladophora. This alga is well known to flourish
 under phosphate- and nitrate-rich conditions. This
 observation fits to the restriction of charophytes,
 which prefer nutrient-poor freshwater to oligohaline
 conditions, to carbonates nearby deltaic deposits.
 Skeletal stromatolites with coarse laminated fabrics,
 that characterize top parts of Hainsfarth-type bio-
 herms, resemble that of Lake Thetis and Lake Clifton
 in Western Australia (Moore 1987, Grey et al. 1990).
 These lakes are alkaline salt lakes with ion ratios similar
 to seawater. From this point of view, the sporadic
 occurrence of one foraminiferal species within
 Hainsfarth-type bioherms is less surprising.
 Youngest bioherms (i.e., Staudigberg-type bioherms)
 are increasingly affected by swamp pedogenesis. The
 lack of calcareous algal tubes in these bioherms
 probably reflects decreasing CaCO3 supersaturationdue to increased humidity and dilution of lake waters,
 finally leading to freshwater deposits with poorly
 calcified charophytes.
 In conclusion, the marginal Ries lake carbonates
 reflect the development from a eutrophic soda lake,
 successively turning into a more Ca2+-rich salt lake
 (with a pronounced seasonality), and finally into an
 oligotrophic Ca2+-rich freshwater lake. This general
 development is superposed by short-term climatic
 fluctuations causing bioherm cycles, and seasonality
 causing annual
 sedimentary rhythms.
 3.4. Lacustrine
 microbialites and
 seawater chemistry
 The algal bioherm suc-
 cession of the Ries basin
 show a replacement of
 non-skeletal stromatolites
 (lacking calcareous cya-
 nobacterial microfossils)
 by skeletal stromatolites
 (with calcareous cyano-
 bacterial microfossils)
 concomitant to the
 successive change from
 soda lake to halite salt
 lake and finally fresh-
 water conditions (Arp
 1995bc). Although a rela-
 tion of cyanobacterial
SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section 227
 calcification to changes in lake water chemistry was
 obvious, the exact mechanisms explaining this
 observation remained unclear.
 Following investigations of biofilm calcification in
 present-day lakes and model calculations demonstra-
 ted that cyanobacterial photosynthesis causes cal-
 careous tubular microfossils only in Ca2+-rich and
 poorly pH-buffered settings (Fig. 11; Arp et al. 2001).
 This relation reflects the solubility product and pH
 buffering by the dissolved inorganic carbon pool.
 Consequently, calcareous cyanobacterial microfossils
 can be used to trace secular changes in seawater Ca2+
 throughout the Phanerozoic when taking into account
 paleo-partial pressure curves for carbon dioxide (Arp
 et al. 2001). In turn, the enigmatic lack of calcified
 cyanobacteria in stromatolite-bearing Precambrian
 sequences can now be explained as a result of high
 dissolved inorganic carbon concentrations (Arp et al.
 2001). With regard to the Ries basin, the formation
 of calcareous cyanobacterial tubes within the skeletal
 stromatolites points to increasing Ca2+-concentrations
 and decreasing alkalinities during the late history of
 the crater lake.
 As a further consequence, secondary Ca2+ release
 from bacterial degradation of biofilm exopolymers
 effectively causes CaCO3 precipitation only when thesimultaneously released CO2 is buffered by highdissolved inorganic carbon concentrations. Therefore,
 the sickle-cell limestones, which reflect calcification
 during biofilm degradation, CO2 release and associatedshrinkage, are characteristic of soda lakes (Arp et al.
 1998, 2003). Similar fabrics related to biofilm degra-
 dation occur in fenestrate microbialites of the Late
 Archaean („cuspate microbialites“ with vertical
 supports and draping laminae; Sumner 1997) and may
 reflect high seawater alkalinities similar to that of soda
 lakes.
 4. Post-Miocene history
 At the end of the Miocene the Ries crater basin was
 completely filled by lacustrine sediments (Bolten &
 Müller 1969). This view is supported by the degree
 of coalification in lignite seams in the clay sequence
 of the research drilling Nördlingen 1973 (Wolf 1977:
 135f.).  Since that time, more than 100 m lacustrine
 sediments have been removed from the central basin,
 largely during the Pleistocene. The central erosional
 plain of the exhumed crater is now at 420-430 m
 above S.L., which is 100-150 m lower than the borde-
 ring hills of the morphological rim.
 Tectonic movements caused a minor tilting of the
 whole region to the northeast. This is evident, e.g.,
 by tilting of Lower Miocene marine Molasse sedi-
 ments (Obere Meeresmolasse) south and southwest
 of the Ries, and highest occurrences of Hydrobia-
 bearing sediments within the Ries basin (see discus-
 sion in Bolten 1977). The tectonic movements may
 have started already late in Miocene times and possibly
 persisted into Pleistocene times (Bolten 1977).
 Meanwhile facies mapping of lacustrine carbonates
 revealed that the bioherm succession at southern Ries
 margin (Armenat 2003) is indeed 25 m higher in
 altitude that at the northern margin (Arp 1995b, Pache
 2000, Peters 2003, Knollmann 2003). Possible first
 movements late in Miocene times may have caused a
 more widespread freshwater deposition at the nor-
 thern Ries margin, while equivalent freshwater
 sediments further south were restricted to pockets of
 emerging spring mound carbonates.
 5. Description of the stops
 Stop 1: Hainsfarth quarry at the Büschelberg (7029
 Oettingen i.Bay., east 44 00 050, north 54 25 000).
 It is the largest and best-known outcrop of dolomitic
 algal bioherms and associated carbonate sands (Fig.
 12). Three adjacent abandoned quarries provide
 exposures more than 330 m long and up to 7 m high
 (Riding 1979). The outcrop has previously been
 described by v. Gümbel (1889, 1891), Wolff &
 Füchtbauer (1976), Bolten (1977), Jankowski (1981)
 and is visited by almost any Ries excursion (e.g.,
 Hüttner et al 1996, Hüttner & Schmidt-Kaler 1999,
 Groiss et al. 2000, Schieber 2002, Höfling 2003).
 However, the carbonates are still commonly termed
 „freshwater limestones“, although most of them are
 dolomites (Klähn 1926, Nathan 1926) deposited in an
 alkaline salt lake (Bolten 1977, Jankowski 1981, Arp
 1995bc).
 The first comprehensible description and drawing of
 the outcrop section has been provided by Riding
 (1979). Several one to two meter thick sequences of
 fused green algal nodules and cones compose loaf-
 shaped dolomitic bioherms up to 15 m across. Bedded
 carbonate sands consisting of peloids, ostracods
 (Standesia risgoviensis) and gastropods (Hydrobia
 trochulus) form irregular channels in between. The
 bioherms are subdivided by discontinuities associated
 with sinter veneers of stalactitic appearance (Fig. 7B),
 delineating pockets and channels of skeletal sands
 (Riding 1979). Thus, these laminated crusts are not
 cyanobacterial stromatolites but lacustrine-vadose
 precipitates due to evaporation-forced calcification of
 biofilms (Arp 1995bc).
 Detailed facies mappings of quarry walls elucidated
 cyclic bioherm growth, discontinuities (erosion,
228 SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section
 dissolution and sinter veneering), and complex timely
 relations between algal carbonates and associated
 skeletal sand of pockets and channels (Fig. 12).
 Complete bioherm cycles, e.g., as seen in basal parts
 of the Hainsfarth quarries, show the following facies
 sequence (Fig. 9):
 (1) First deposits are palustrine limestones, either rich
 in bioherm clasts of previous cycles or extraclasts of
 the pre-Miocene basement (e.g., the breccia of Ju-
 rassic limestones at the basis of the Hainsfarth sec-
 tion). Common are root traces and moulds of Cepaea
 sylvestrina.
 (2) Bioherms start with nodules and indistinct cones
 of Cladophorites-bafflestones rich in mesogastropods,
 dissolution voids and laminated sinter crusts indicating
 common subaerial exposure events in the wave-
 exposed eulittoral (Fig. 7B).
 (3) Cones of Cladophorites-framestone with abun-
 dant ostracods, few dissolution voids and thin sinter
 veneers represent maximum flooding and shallow
 sublittoral conditions (Fig. 7A).
 (4) A further bed of nodular Cladophorites-
 bafflestones with mesogastropods and dissolution
 voids follows, with increasingly thick sinter veneers.
 This points to a decrease in lake level and an increasing
 abundance of subaerial exposure events.
 (5) Skeletal stromatolites formed during eu- to supra-
 littoral conditions are locally developed on top of the
 cycle (Fig. 7D).
 (6) Finally, the cycle ends with a prolonged subaerial
 exposure, erosion and formation of sinter veneers.
 The next cycle starts with palustrine limestones with
 bioherm fragments, root voids and Cepaea sylvestrina
 in erosional pockets and fissures of the completed
 bioherm cycle (Fig. 9). Further three, reduced, cycles
 form the main part of the bioherms in Hainsfarth,
 followed by two undulating bioherm beds at the out-
 crop top (Fig. 12). These cycles show an increasing
 portion of skeletal stromatolites, which become
 widespread at the already poorly exposed outcrop rim.
 Strikingly, the contact between algal bioherm car-
 bonates and skeletal sands is always discontinuous
 and marked by sinter veneer (Fig. 12). This means
 that skeletal sands are always younger than the
 adjacent bioherm carbonate. Facies mapping of
 quarry walls shows that ostracod-rich framestones
 correlate with ostracod-rich pocket fillings 0.3 to 1
 m below them, suggesting a syndepositional relief.
 Thus, ostracods were deposited in already sinter-
 veneered pockets and fissures of previous bioherm
 cycles. The depth of erosional pockets filled with
 palustrine carbonates indicate long-term lake level
 fluctuations in the order of 1 to 2.5 m (Fig. 9), while
 seasonal fluctuations may have been less (Arp
 1995bc).
 Stop 2: Breitenlohe quarry (7029 Oettingen i.Bay., east
 43 93 600, north 54 27 400).
 Palustrine carbonates are exposed with up to 2,4 m
 thickness in this abandoned field quarry 400 m SSE
 of Breitenlohe (Arp 1995bc). The yellow to white-
 grey, instinct bedded limestones show dissolution-
 enlarged root voids (1-3 mm diameter), complex
 desiccation cracks (Fig. 13) and scattered pulmonate
 gastropods (Cepaea sylvestrina). Coarse bedding is
 caused by less resistant limestone layers or inter-
 calations of porous intraclasts.
 The microfacies is characterized by pedogenically
 neoformed nodule-ooid-wackestones with root voids
 and animal burrows. Only at the outcrop base pedo-
 genically neoformed nodule-wackestones with skew
 planes are developed (Fig. 13). They show grey
 micritic patches due to soft pebbles and bioturbation
 („striotubules“).
 Stop 3: Fields west of Breitenlohe (7029 Oettingen
 i.Bay., east 43 93 100, north 54 27 750).
 The fields west of the village Breitenlohe constitute
 the largest relict occurrence of final freshwater
 deposits of the Ries crater lake. A detailed map and
 description of this occurrence has been provided by
 Bolten (1977).
 Lower parts of the freshwater deposits are ochre-
 coloured calcareous marls with a diverse freshwater
 gastropod fauna, described by Bolten (1977) from a
 temporary trench. On top of them, white-grey
 weathered, loose limestone plates with abundant
 Gyraulus kleini follow, which are widely distributed on
 the fields (Fig. 10F). Marly limestones with aragonitic
 shells of Gyraulus kleini, Radix socialis dilatata and
 Planorbarius cornu mantelli are much more rare. At the
 northern end of the freshwater limestone occurrence,
 a narrow E-W trending strip with intraclast-rich marly
 micrites containing disarticulated carapaces of the
 tortoise Testudo sp. was found (Arp 1995bc).
 Stop 4: St. Georg’s church Utzwingen (7029 Oet-
 tingen i.Bay., east 43 90 175, north 54 23 675).
 This fortified church is largely built of algal carbonates
 from quarries nearby. The porous but rigid dolomitic
 algal bioherm carbonates once formed one of the few
 suitable building stones in the Ries area, aside from
 the partially melted impact breccia suevite. There are
 no documents on the age of this church, but remains
 of ancient, now closed windows suggest a construc-
 tion in the twelfth to late thirteenth century. The
SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section 229
 Fig. 13. Complex desiccation cracks in palustrine limestones.
 A. Skew planes. Breitenlohe quarry.
 B. Craze planes with inverse gradation. N of Staudigberg.
 C. Curved planes. N of Staudigberg. From Arp (1995b).
 Fig. 14. Lithologic log of the fluviodeltaic conglomerates exposed in
 the road-cutting Ulrichsberg W of Maihingen.
 present large windows probably originated in baroque
 time. More recently, Pleistocene travertine was used
 for repair of the walls at few places. At the small
 parking lot in front of the church, a basin made of
 dolomitic algal bioherm carbonates can be seen at a
 water pump. The cutting planes excellently show
 nodular Cladophorites-bafflestones with laminated
 sinter veneers and pockets in-between. In addition,
 several Christian crosses cut from algal bioherm
 dolostones excavated during a pipeline construction
 near Maihingen are visible in this rural commune.
 Stop 5: Road-cutting Ulrichsberg (7028 Unter-
 schneidheim, east 36 09 000, north 54 22 000).
 The road-cutting exhibits 4 m of bedded, coarse
 conglomerates and greenish-grey, clayey sandstones
 resting on impact-brecciated crystalline basement
 rocks (Fig. 14, 15). These conglomerates consist of
 cm-sized crystalline rock pebbles and are interpreted
 as deposits of a deltaic inlet into the Ries lake. The
 conglomerates form lenticular beds with a poorly vi-
 sible cross-stratification directed to southern direc-
 tions, as formerly exposed in a gravel pit north of the
 road cutting (Groiss 1974). Few, thin beds of light-
 grey, sandy dolomites with root traces and the meso-
 gastropod Hydrobia trochulus are intercalated, indicate
 a transition to dolomitic carbonate sands of the lake
 shore. To top of the succession, the transition to dolo-
 mitic algal bioherms, which cover the Ulrichsberg, is
 poorly exposed (Fig. 14, 15). Noteworthy is an 80
 cm-thick conglomeratic bed near the outcrop basis,
 which shows dm-sized algal bioherm fragments,
 Cladophorites-tufts and Hydrobia trochulus. These
 components demonstrate that bioherm formation was
 contemporaneous with fluviodeltaic influx.
230 SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section
 Fig. 15. Geological cross-section of the Mellerberg-Ulrichsberg area showing the position of fluviodeltaic conglomerates, algal bioherms and
 spring mounds.
 Stop 6: Mellerberg (7028 Unterschneidheim, east 36
 09 450, north 54 21 675).
 The Mellerberg („Etter“) is a small hill southwest of
 the road-cutting Ulrichsberg, already located within
 the Ries plain (Fig. 15). It exhibits dolomitic algal
 bioherms (Wolff & Füchtbauer 1976) associated with
 fossiliferous pack/grainstones (Strandesia risgovien-
 sis, Hydrobia trochulus) and non-skeletal stromatolites.
 However, new exposures during the construction of
 a cycleway (alongside the road from Maihingen to
 Birkhausen) demonstrated that this Adlersberg-type
 algal bioherm exhibits a spring mound in its centre.
 Sickle-cell limestone boulders excavated during this
 roadwork are now placed beside of the cycleway.
 Stop 7: Wallerstein castle rock (7128 Nördlingen, R
 36 08 450, H 54 17 430)
 This hill is one of the largest spring mounds in the
 Ries basin, forming a conspicuous rock upon the
 crystalline ring structure of the crater (Fig. 2). Cry-
 stalline rocks of the pre-Miocene basements have been
 drilled below Strandesia- and Hydrobia-rich car-
 bonates 670 m west of the castle rock (Nathan 1957).
 The castle rock attains a height of 495.6 m of above
 sea level, thereby surpassing the surrounding Ries plain
 by 70 m. Approximately 30 m of „travertine“ are
 exposed due to quarrying during the twelfth to nine-
 teenth century (Diemand, 1898).
 Most parts of the Wallerstein are composed of sickle-
 cell limestones (Reis 1926), i.e., highly porous lime-
 stones characterized by irregular, wavy laminae enclo-
 sing lenticular to sickle-cell-shaped voids (Fig. 5C, 16).
 At the northside base of the castle rock, such lime-
 stones form the cores of steep-sided pinnacles up to
 4.5 m height, which are veneered by dm-thick throm-
 bolitic crusts of clotted fabric (Fig. 5B, 16). On top
 of these pinnacles, a bed of more or less horizontal,
 platy non-skeletal stromatolites is developed, indica-
 ting lake-level lowstand conditions. Saline conditions
 are indicated by pseudomorphs of unidentified mono-
 clinic-prismatic evaporite minerals. Non-skeletal stro-
 matolites as well as thrombolites are rich in faecal
 pellets resembling that of the modern brine shrimp
 Artemia. Locally sickle-cell limestones form m-sized
 columns of stromatolitic appearance, e.g., at the stair-
 way to the summit (Fig. 5C). At the eastern side below
 the summit, curtains of thin stalactites occur, reflecting
 early subaerial exposure already in Miocene times. On
 top of the castle rock, concentric structures of
 pinnacle cross-cuttings are visible.
 Initially, spring mounds were considered as hot sp-
 ring deposits of a Ries volcano (e.g., v. Gümbel 1870,
 Klähn 1926, Seemann 1935, 1941). However, after
 uncovering the impact origin of the Ries, artesian
 springs were favoured as cause of the „travertine“
 deposits. Wolff & Füchtbauer (1976) suggested that
 the Ries „travertines“ are largely inorganic sinter lime-
 stones formed in early the history of the Ries lake by
 CO2-degassing at subaerial springs discharging atconcentrically arranged shear-planes of the inner
 crystalline ring.
 Later, Bolten (1977) and Bolten & Gall (1978) pro-
 posed that the Wallerstein „travertines“ formed, after
 an initial subaerial phase, submerged at a calcareous
 artesian spring discharging into a brackish lake. CO2degassing, rise in temperature and cyanobacterial
 activity were considered as driving mechanisms of
 CaCO3 precipitation, although a comparison with tufapinnacles of soda lakes was already drawn (Bolten
 1977: 78). However, studies in present-day soda lakes
 indicate that tufa pinnacles similar to that of the Ries
 lake result from mixing of Ca2+-supplying subla-
 custrine spring water and alkaline lake water (Arp et
 al. 1998, 1999). With regard to the Wallerstein, this
 interpretation is supported by 87Sr/86Sr isotope ratios
 that are shifted towards that of crystalline basement
 rocks, if compared to 87Sr/86Sr isotope ratios of algal
 bioherms (Fig. 16; Pache et al. 2001). On the other
 hand, stable oxygen and carbon isotope analyses
SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section 231
 Fig. 16. Schematic section of the spring mound Wallerstein. Below, covariation plots of d18O vs. 87Sr/86Sr and d13C vs. 87Sr/86Sr of spring mound
 carbonates in comparison to algal bioherms and Upper Jurassic limestones are shown. Model curves demonstrate binary mixing of lake water
 and spring water from crystalline basement rocks (dashed lines) and fluid-rock interaction within the upwelling groundwater flow (solid
 lines). From Pache et al. (2001).
 indicate that the CO32- of the primary spring moundcarbonates is largely derived from the lake water (Fig.
 16).
 The cyanobacterial biofilm, at first loosely CaCO3impregnated in such mixing zone, become succes-
 sively lithified during shrinkage, bacterial degradation
232 SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section
 Fig. 17. The spring mound Alerheim castle rock located on the crystalline ring of the impact crater.
 Fig. 18 A and B. Fluviodeltaic sands showing foreset beds are overlain
 by Hainsfarth-type dolomitic algal bioherms associated with peloidal
 carbonate sands. Sand pit Megesheim.
 and secondary Ca2+ release from the biofilm exopo-
 lymers (Fig. 6; Arp et al. 1998). This mechanism
 sickle-cell fabric formation is only effective in highly
 alkaline water, i.e., soda lakes, buffering the simul-
 taneously released CO2 (Arp et al. 2003).
 The Wallerstein carbonates are largely very poor in
 fossils, although avian eggshells and one tortoise have
 been reported (Deffner & Fraas 1877, Bolten 1977,
 Kohring & Sachs 1997). Such fossils appear to be
 restricted to pocket and fissure fillings of a freshwater
 phase younger than the true spring mound carbonates.
 Stop 8: Alerheim castle rock (7129 Deiningen, east
 43 98 330, north 54 11 730)
 Together with the Wennenberg, Steinberg and Hahnen-
 berg nearby, this conspicuous hill is part of the
 crystalline ring structure in the southeastern crater
 sector (Fig. 17). Located vis-à-vis the Wallerstein on
 the opposite site of the crater, the castle rock is another
 example of a large spring mound formed on a
 sublacustrine spring discharging from crystalline
 basement rocks (Bolten 1977). Similar to the Wallers-
 tein castle rock, it is largely composed of sickle-cell
 limestones and associated speleothems. In addition,
 Nathan (1926) mentioned thin-bedded carbonates with
 ostracods from the western foot of the Alerheim castle
 rock. The castle rock is private property and to date
 poorly investigated.
 Stop 9: Megesheim sand pit (7029 Oettingen, east 44
 01 750, north 54 24 500)
 The outcrop shows deltaic clastics of the Ries basin
 margin (Fig. 18). The brownish quartz sands and
 friable sandstones are part of the so-called „delta of
 Trendel“ (Bolten & Müller 1969). Years ago, almost
 8 m of these deltaic sands were exposed in the sand
 pit (Chao et al. 1987). Large-scale foreset beds dipping
 10° to 30° NNW are still visible (Fig. 18A) and point
 to a transport from crystalline rock areas further
 southeast (Chao et al. 1987, Hüttner et al. 1996).
 Indeed, layers of centimetre-sized crystalline rock
 pebbles are common. Upper Jurassic limestone clasts
 and kaolinized impact glass fragments are much more
 rare. In addition, few chalky, white-grey micrite beds
 up to 5 cm thick are intercalated.
 After a discontinuity, one meter of dolomitic Hains-
SEDIMENT 2006: 21th Meeting of Sedimentologists / 4th Meeting of SEPM Central European Section 233
 farth-type algal bioherms and associated carbonate
 sands form the top part of the outcrop (Fig. 18B).
 Bioherms are composed of sinter-veneered Cladopho-
 rites-bafflestones, while carbonate sands largely
 consist of peloids. These peloid-packstones contain
 intraclast layers, abundant Hydrobia trochulus and
 ostracods. Besides of Strandesia risgoviensis, rare
 Eucypris sp. and Cypridopsis have been found, too
 (Janz in Arp 1995b). Locally, root traces have been
 observed.
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