Skeletal Accumulations Research Articles

Taphonomy and depositional history of the Southfork Quarry (Cypress Hills Formation, late Eocene) in southwestern Saskatchewan, Canada

The Eocene to Miocene Cypress Hills Formation (CHF) spans 28 million years and forms the conglomeratic caprock of the Cypress Hills and Swift Current plateaus in southwestern Saskatchewan and southeastern Alberta. These stacked fluvial, floodplain, and lacustrine deposits preserve the only high latitude, non-polar mammalian fossil assemblage (Uintan to Hemingfordian land mammal stages) in Canada. The Quarry is the oldest CHF (Chadronian 1, late Eocene) site documented in the Cypress Hills region north of the town of Eastend. The Quarry was originally discovered in 1962, after bones were found to be eroding out of the base of a road cut north of the ghost town of South Fork on the southeastern flanks of the Cypress Hills. Numerous field campaigns have resulted in the collection of fossils from a multitaxonomic bonebed. This paper presents a detailed sedimentologic, paleontologic, and taphonomic study to establish a depositional environment framework of the Southfork Quarry. This site was deposited at the onset of the Eocene–Oligocene climate transition, a critical time of climate change during the Paleogene. Six facies and two facies associations are characterized for the Quarry, shifting from a braided-fluvial system to a debris flow-filled incised channel. Patterns of skeletal accumulations and bone surface modification indicate that the assemblage accumulated over a significant interval of time in different depositional environments. This study provides critical insight into environmental shifts driven by climate change and relates these findings to a broader understanding of the Eocene–Oligocene transition in North America.

Middle Devonian Skały Formation in the Holy Cross Mountains (Poland) – formal description and subdivision based on new field data

The well-known fossiliferous and lithologically variable clay-carbonate series in the Łysogóry Region (northern part of the Holy Cross Mts, central Poland), enclosed between the Middle Devonian Amphipora dolomites and limestones (Kowala Formation) and siliciclastics (Świętomarz Beds), is defined formally as the Shaly-Calcareous Skały Formation. This Upper Eifelian to Middle Givetian, ca. 250–280 m thick unit, consists of marly and clay shales, interbedded many times with various limestone types (including encrinite and biohermal varieties), as well as with marls and siltstones. Its diagnostic feature is the presence of variable skeletal accumulations, formed by exceptionally numerous, well-preserved and diverse macrofauna (including brachiopods, corals, crinoids, bryozoans), described since the 19th century. The stratotype is located in the eastern slope of the Dobruchna stream near the Skały village and belongs to the Silurian to Upper Devonian Grzegorzowice-Skały section. Compared to the previously used term, Skały Beds sensu Pajchlowa (1957), the lower boundary is redefined, owing to a new exposure in the active Skała Quarry, and placed higher, at the base of the famous brachiopod shales (set XIV of Pajchlowa), instead of the formerly accepted lower boundary at the base of set XIII. Set XIV is formally distinguished as the Dobruchna Brachiopod Shale Member. The higher part of the Skały Fm (sets XV–XXVA) is not subdivided further, as the poorly exposed succession, including in particular the type area, precludes a more accurate recognition of lithological variability. The upper boundary of the Skały Fm is placed at the top of set XXV sensu Pajchlowa (1957), corresponding to the boundary between subsets XXVA and XXVB sensu Malec and Turnau (1997). A hypostratotype of the upper boundary is selected in the outcrop M0 at Miłoszów, 2.5 km westwards from the type section, allowing recognition of the diachroneity of lithological change defining the transition from the Skały Fm to Świętomarz Beds. A borehole situated in a key location would be an obvious next step in the further elucidation of the stratigraphic sequence of the Łysogóry Region.

Integrated diagenetic and sequence stratigraphy of a late Oligocene–early Miocene, mixed-sediment platform (Austral Basin, southern Patagonia): Resolving base-level and paleoceanographic changes, and paleoaquifer characteristics

A condensed (~20-m-thick) marine transgressive-highstand succession comprises the upper San Julián Formation (upper Oligocene–lower Miocene) of the northern retroarc Austral Basin, southern Patagonia. Mixed-sediment facies identify a shelf-interior setting, part of an overall warm-temperate regional platform of moderate energy. Giant oyster-dominated skeletal–hiatal accumulations along the maximum flooding surface and forming high-energy event beds in the highstand succession preserve relict micrite in protected shelter porosity, and identify periods of reduced sediment accumulation. The stratigraphic distribution of marine-derived glaucony and diagenetic carbonates is spatially related to sequence development. Depositional siderite coincides with prominent marine transgression, defining transient mixing of marine and meteoric waters across coastal-plain deposits. Chemically evolved autochthonous glaucony coincides with periods of extended seafloor exposure and transgressions that bracket the marine succession, and within the oyster-dominated skeletal accumulations. Seafloor cement, likely once magnesian calcite, formed in association with an encrusting/boring biota along the maximum flooding surface in concert with incursion of cool (11–13°C) water. The cement is present locally in skeletal event beds in the highstand succession suggesting a possible association with high-order base-level change and cooler water. As the highstand succession coincides with elevated global sea level in the late Oligocene–early Miocene, the locally marine-cemented glauconitic skeletal event beds in the highstand succession may identify higher order glacio-eustatic control. Local stratal condensation, however, is best explained by regional differences in basement subsidence. In the burial realm, carbonate diagenesis produced layers of phreatic calcrete coincident with skeletal-rich deposits. Zeolite (clinoptilolite-K) cement is restricted to the lowermost marine transgressive interval probably due to initial elevated metastability of reworked weathered silicates. Clay (illite)-cement is restricted to siliciclastic-rich intervals wherein skeletal carbonate did not buffer pore-water pH. Diagenetic carbonate geochemistry (Sr, Na, and δ18O and δ13C) shows that, with burial, the transgressive and highstand system tracts developed as distinct paleoaquifers resulting from different proximities to meteoric recharge zones.

Taphonomic traits of clay-hosted early Cambrian Burgess Shale-type fossil Lagerstätten in South China

The Chengjiang and Guanshan fossil Lagerstätten of Yunnan represent key fossil deposits to understand and reconstruct early metazoan evolution. Despite extensive studies on the systematics of taxa from these Lagerstätten, the major mechanisms of exceptional soft-tissue preservation at these fossil Lagerstätten have remained controversial. The Chengjiang and Guanshan fossil Lagerstätten represent typical Burgess Shale-type deposits, influenced by late alteration and weathering that led to some major transformations in clay and iron minerals, as well as dissolution of dolomite and skeletal biominerals. The detailed geochemical screening of various fossil groups, such as brachiopods, priapulids, arthropods, and sponges, indicates a more complex preservational history than hitherto supposed. Besides oxidation processes during sub-recent/recent weathering, clay mineral formation and/or alteration, carbonization, phosphatization, and pyritization played key roles in the preservation of various metazoans. The Chengjiang fossil Lagerstätte is characterized by the existence of clay-dominated background beds (BGBs) and event beds (EBs). Fossil preservation varies considerably in the two different depositional settings, with soft-tissue preservation occurring preferentially in EBs, whereas skeletal accumulations are typical of BGBs.The samples were analyzed and classified into three groups with different degrees of alteration and fundamentally different chemistries. The specific clay mineral composition shows increasing chlorite and goethite, decreasing kaolinite, 2M1-mica, and dolomite, and a rise in the chemical alteration of mudstones. It also indicates slight variations between the primary types of beds (EB, BGB), hinting at variations in the source of clastics or the hydrochemical environment during the deposition of strata. Furthermore, the geochemical paleoredox proxy data indicate that dysoxic or anoxic conditions were not permanently present during the deposition of EBs and BGBs in the Chengjiang and Guanshan fossil Lagerstätten.We consider that rapid deposition in finest claystones had a greater influence on the preservation of highly volatile tissues than the presence of bottom-water anoxia, even though bottom-water anoxia and microbial sealing may further increase the probability of soft-tissue preservation. Using different geochemical proxies such as various redox indicators and iron speciation, statistics on framboid sizes, classical geological observation, and clay mineralogy, we show that iron mineral precipitation onto organic carcasses of the Chengjiang-type fossils is mostly confined to later diagenetic processes and thus is not crucial for the early fixation of easily decayable tissues in Burgess Shale-type faunas.

Designing an Upper-Level Vertebrate Paleontology and Taphonomy Course for Undergraduate Geoscience Majors

Designing and teaching a vertebrate paleontology course for geoscience majors presents several challenges. Students often come to the course with limited or nonexistent biology backgrounds, and therefore may begin the semester anxious about their ability to master course material. Moreover, students may be skeptical about the value of learning vertebrate skeletal anatomy for their future careers as geoscientists. Vertebrate Paleontology and Taphonomy is an upper-level elective for geoscience majors that was intentionally designed to allow students to develop a basic understanding of vertebrate osteology for themselves before focusing on formational histories of vertebrate skeletal accumulations in geological context. The course relies heavily on hands-on exposure to modern and fossil skeletal material, field trips to local museum galleries and collections, cooperative laboratory activities and projects, and analysis of real-world data sets. Students work together with one another and the instructor to make observations on vertebrate fossils, analyze their own data and data from the primary literature, and interpret taphonomic histories of actual vertebrate assemblages. This structure makes success in the course less about ‘learning vertebrate paleontology’ and more about using vertebrate paleontology and taphonomy as a tool to become effective practicing geoscientists.

Quantitative comparisons and models of time-averaging in bivalve and brachiopod shell accumulations

The variation in time-averaging between different types of marine skeletal accumulations within a depositional system is not well understood. Here we provide quantitative data on the magnitude of time-averaging and the age structure of the sub-fossil record of two species with divergent physical and ecological characteristics, the brachiopodBouchardia roseaand the bivalveSemele casali.Material was collected from two sites on a mixed carbonate-siliciclastic shelf off the coast of Brazil where both species are dominant components of the local fauna.Individual shells (n= 178) were dated using amino acid racemization (aspartic acid) calibrated with 24 AMS radiocarbon dates. Shell ages range from modern to 8118 yearsb.p.for brachiopods, and modern to 4437 years for bivalves. Significant differences in the shape and central tendency of age-frequency distributions are apparent between each sample. Such differences in time-averaging magnitude confirm the assumption that taphonomic processes are subject to stochastic variation at all spatial and temporal scales. Despite these differences, each sample is temporally incomplete at centennial resolution and three of the four samples have similar right-skewed age-frequency distributions. Simulations of temporal completeness indicate that samples of both species from the shallow site are consistent with a more strongly right-skewed and less-complete age-frequency distribution than those from the deep site.We conclude that intrinsic characteristics of each species exert less control on the time-averaging signature of these samples than do extrinsic factors such as variation in rates of sedimentation and taphonomic destruction. This suggests that brachiopod-dominated and bivalve-dominated shell accumulations may be more similar in temporal resolution than previously thought, and that the temporal resolution of multi-taxic shell accumulations may depend more on site-to-site differences than on the intrinsic properties of the constituent organisms.

Facies and Stratigraphic Anatomy of a Temperate Carbonate Sequence (Capo Colonna Terrace, Late Pleistocene, Southern Italy)

Abstract Accurate interpretation of ancient non-tropical carbonate systems requires the development of appropriate facies and sequence stratigraphic models. These models should account for the marked differences in sedimentation style among non-tropical carbonates formed under different energy settings. This paper presents data and a conceptual model of the sequence architecture of non-tropical carbonates of moderate energy environments based on the late Pleistocene deposits of Capo Colonna marine terrace (Calabria, southern Italy). These deposits have the advantage of being continuously exposed along a proximal–distal transect across the paleo-shelf and their geometry has not been disturbed by later tectonic activity. They consist of a transgressive–regressive (T–R) cycle whose genesis can confidently be related to one of the interstadial glacio-eustatic fluctuations of marine isotope stage (MIS) 5. Eight different mixed carbonate–siliciclastic and carbonate facies are present in the studied sequence. Of these, biogenic-dominated facies are invariably made up of skeletal remains of light-independent organisms plus or minus red calcareous algae (the heterozoan association). The relatively young age of the deposits allows calibration of the paleoecological significance of observed facies by direct comparison with modern analogues from the Mediterranean Sea. Four laterally persistent discontinuity surfaces were identified along the transect, and interpreted, from bottom to top, as (1) a basal ravinement surface, (2) a submarine hardground locally acting as a substrate for algal boundstones, (3) a regressive surface of marine erosion, and (4) a surface of subaerial exposure. Sequence stratigraphic analysis indicates that the preserved T–R cycle is asymmetric, with a thicker regressive portion. Highstand deposits can be subdivided into a shore-connected sandy prism and more distal autochthonous and parautochthonous skeletal accumulations and algal boundstones. During forced regression, a thin, sharp-based, strongly progradational unit of calcarenites was deposited above the regressive surface of marine erosion. The deposits of Capo Colonna terrace document the response of non-tropical, moderate-energy carbonate ramps to sea-level fluctuations during icehouse conditions. Their architecture can facilitate the interpretation of other successions from similar settings and provide an end member to be compared and contrasted with high-wave-energy cool-water carbonates.

Evidence for gregarious behavior and age segregation in sauropod dinosaurs

Both body fossil and ichnofossil evidence for gregarious behavior in sauropod dinosaurs is examined. Some localities suggest that herds were partitioned on the basis of age, whereas other sites reveal groups consisting of both adult and juvenile/subadult individuals. Two skeletal accumulations showing evidence of age segregation are examined in detail. The Mother's Day Quarry in the Upper Jurassic Morrison Formation of Montana contains the remains of several immature diplodocoid sauropods. An assemblage in the Upper Cretaceous Javelina Formation of Big Bend National Park in Texas consists entirely of juvenile Alamosaurus. Both the Mother's Day and Big Bend assemblages are interpreted here as remnants of age-segregated herds. The differences between skeletal accumulations composed entirely of immature animals and mixed-age ichnological assemblages imply that herd composition was variable. When age segregation is recognized, in both fossil and modern taxa, it is thought to minimize the fitness costs related to behavioral synchronization within social groups. Age segregation of herds also contraindicates extended parental care as typical of at least some sauropod taxa.

Spatial competition among clonal organisms in extant and selected paleozoic reef communities

Occurrences of densely packed benthic organisms in extant reefs are of two types: 1) live-live interactions, where two living organisms interact, and 2) live-dead associations, where only one is alive and uses the other as a substrate. The latter are common in reef deposits due to biostratinomic feedback, i.e. dense skeletal accumulations provide hard substrates for clonal recruitment, thus facilitating greater frequency of live-dead encounters than in lower biomass level-bottom communities dominated by solitary organisms. Differentiating between these two types in ancient reefs is difficult, often impossible.

Human digestive effects on a micromammalian skeleton

This study describes the results of an experiment involving the consumption of a skinned, eviscerated, and segmented insectivore by an adult human male. Bone remains from recovered faecal contents are examined for skeletal element representation, breakage and digestive damage. Detailed examination of each category suggests severe skeletal attrition which is comparable to, and at times in excess of, the damage exhibited in microvertebrate skeletal accumulations originating from the scats of small mammalian carnivores.

Using numerical models to evaluate the consequences of time-averaging in marine fossil assemblages

Taphonomic studies of marine fossil assemblages have achieved new levels of sophistication in the past several years, but field-based investigations of time-averaged skeletal accumulations continue to suffer from an unavoidable problem: to date, a time machine has yet to be invented that would permit the researcher to observe directly the formation of the assemblage under study. Even when working with Recent subfossil accumulations on the sea floor, where it is theoretically possible to view the activity of (possibly myriad) taphonomic processes, practical considerations prevent the extensive, day-to-day monitoring that would be required to “bear witness”, in a kinetic sense, to the formation of skeletal assemblages. At best, we are sometimes provided with opportunities for comparative glimpses of skeletal accumulations before and after events of potential taphonomic significance, such as hurricanes (e.g., Miller et al., 1992).

Organisms and sediments: relationships and applications

The sixth Lyell meeting of the Geological Society was held on 13 February 1992. Major themes included advances in the study of trace fossils with emphasis on ichnofabrics, and the use of skeletal accumulations (shell and bone beds), both in elucidating environmental change, and in sequence stratigraphy. Of the 12 papers read at the meeting, seven are published in this thematic set. The papers presented cover topics which Lyell did not himself discuss at any length, with the exception of his classic study on the Temple at Puzzuoli. Nevertheless the theme of the meeting was aptly expressed by him: ‘Arrangements of fossils in strata: Each stratum in fact however far it may now lie beneath the surface was once in the state of shingle, loose sand or soft mud at the bottom of the sea, in which shells and other bodies easily became enveloped. By attending to the nature of these remains we are often enabled to determine whether deposition was slow or rapid, whether it took place in a deep or shallow sea, near the shore or far from land, and whether the water was salt, brackish or fresh’ (Elements of Geology, Chapter 3). As well as honouring Lyell it was befitting to remember Robert W. Frey (University of Georgia, Athens) who died on 1 January 1992 following a long fight against cancer. Much of his considerable research output was directly in the fields covered at the meeting. Over the past 20 years he pioneered the study of animal-sediment

The Relationship between Sediment Fabric and Planktonic Microfossil Taphonomy: How Do Plankton Skeletons Become Pelagic Ooze?

Taphonomic studies have provided valuable information on depositional and post-burial conditions of sediments. Deep sea oozes, by definition, are predominantly composed of the skeletal remains of pelagic organisms. These skeletal accumulations are similar in many ways to those of shallow-marine invertebrates, which have been the typical subjects of taphonomic analysis. We consider the problem of obtaining meaningful information from taphonomic analysis of pelagic microfossils, and the relationship between microfossil taphonomy and sediment fabric. Using examples from the biosiliceous oozes of Ocean Drilling Program (ODP) Site 751, we evaluate taphonomic concepts developed by previous workers for analysis of macrofossil skeletal accumulations and relate these to out observations of sediment fabric

How good was the fossil record? Clues from the Californian Pleistocene

The living members of 113 families of bivalves and gastropods of the Californian Province include 698 species living at shelf depths, of which 538 or 77% are known as Pleistocene fossils from the same region; another 113 fossil species are extralimital, and 98 are extinct. Living species not found as fossils are chiefly rare today, and/or minute, fragile, and/or from deeper shelf habitats. Sampling of the Pleistocene record has been biased towards shallow-water assemblages. Fragile and minute forms are probably underrepresented in the record. Rare forms, however, are still appearing as new studies are conducted, and many rare species are yet to be discovered. At least 85% of durably skeletonized living species may have been captured in the record. It is probable that most durably skeletonized invertebrate species were represented in lithostratigraphic units throughout the Phanerozoic, but that this record is lost owing to erosion, burial, and destruction of skeletons in situ. The bulk of the marine invertebrate fossil record does not represent a series of unusual skeletal accumulations, but rather the preserved remnants of an excellent original record formed through ordinary though episodic processes.

Palaeobiological and sedimentological implications of fossil concentrations

Concentrations of fossil hardparts are common features of the stratigraphical record and are preferred collecting sites for most palaeontological data. Nonetheless, most investigations into the nature of the fossil record have analysed the biasing effects of selective hardpart transport and destruction1,2 rather than the consequences of the concentration process itself. Genetic classification is discouraged by the diverse origins of skeletal accumulations, which range from predator gastric residues to shelly shoals and biostratigraphically condensed deposits; concentrations can thus form over time intervals of a few minutes to hundreds of thousands of years. I show here that the close association of shell beds with stratigraphical discontinuities in Miocene shallow marine deposits of Maryland3,4 provides the basis of a model of skeletal accumulation cast entirely in terms of changes in net sedimentation. This simple sedimentological model is a surprisingly powerful predictor of post-mortem bias and ecological composition of fossil assemblages, suggesting that fossil-rich and fossil-poor strata are qualitatively different, both as repositories of palaeontological information and as settings for biotic interactions. Moreover, the apparent primary importance of rates of sedimentation in skeletal accumulation—despite emphasis usually placed on rates of hardpart input—suggests a new approach to inferring the detailed dynamics of sediment deposition and erosion in the formation of stratigraphical sequences.

Biofabrics as Dynamic Indicators in Nummulite Accumulations

Biofabric interpretation of skeletal accumulations consisting of biologically standardized nummulite tests (larger foraminifera) is based upon (1) the ratio of small-bodied A-forms (megalospheric) and large-bodied B-forms (microspheric), and (2) on their packing and imbrication features. In the Eocene Mokattam Formation of Egypt, Nummulites gizehensis occurs in limestones with four basic biofabrics. In wackestones, the nummulites float throughout the matrix, and A-forms strongly dominate over B-forms, approximating the ratio reported for undisturbed assemblages. Packstones, however, reflect various degrees of sedimentary reworking and winnowing of the original substratum by physical processes. For packstones with edge-wise imbrication and A-forms strongly dominating over B-forms, in situ winnowing of fine-grained matrix by waves can be inferred. In contrast, packstones composed of imbricated B-forms suggest hydraulic sorting and selective removal of A-forms by currents. Packstones composed almost entirely of A-forms represent transport-sorted allochthonous deposits. Biofabric analysis thus allows us to understand better the dynamics of skeletal accumulations such as nummulite banks, whose commonly complex and extensive buildups can form hydrocarbon reservoirs.

Origin of brachiopod‐bryozoan assemblages in an Upper Carboniferous limestone: importance of physical and ecological controls

Skeletal accumulations in the Upper Carboniferous Bird Spring Formation of southern Nevada have yielded new information regarding ecological succession in the fossil record and the development of shell concentrations. Through hydraulic baffling processes, a community dominated by cndobyssate bivalves and strophomcnid brachiopods was locally supplanted by a community composed primarily of bryozoans, compositid brachiopods, and various epizoans. The general trophic and diversity changes resemble those expected during ‘short-term ecological succession’ but are due for the most part to the sedimentological and hydraulic regime. In view of this evidence, previous paleontologic examples of ‘autogenic’ succession arc re-interpreted as having been primarily controlled by the physical environment. Upper Carboniferous, Brachiopoda, Bryozoa, paleoecology, ecological succession, epifauna.

A trough cross‐stratified glaucarenite: a Cambrian tidal inlet accumulation

ABSTRACTThe upper part of the Riley Formation, Cambrian of central Texas, is primarily composed of a sequence of thoroughly trough cross‐stratified deposits. The dominant lithologies range from fossiliferous glaucarenite to highly glauconitic bio‐sparrudite. These cross‐stratified deposits accumulated within a tidal inlet and associated lagoonal tributary and distributary channels. Tidal inlet‐fill strata are underlain by shallow, open marine oosparites and biomicrites and are overlain by parallel bedded glaucarenites which accumulated as part of a barrier island complex. The parallel bedded deposits exhibit large scale, gently inclined strata, ripple cross‐stratification, and a minor amount of vertical burrows.Some glaucarenite units within the tidal inlet‐fill have local concentrations of skeletal material, primarily trilobite carapaces. These concentrations are most abundant in the bottoms of troughs. Cementation by bladed to fibrous spar between the carapaces has resulted in the nodular appearance of these skeletal accumulations. Calcite clasts, with relict evaporite textures, occur within the carbonate nodules and surrounding glaucarenite. These clasts were eroded from the shallow subsurface of the barrier island as the tidal inlet migrated. The presence of the former evaporite clasts attest to an arid climate at the time of their formation.

Original and Secondary Pores in Sedimentary Reservoir Rocks: Relation to CaCO3 Cements: ABSTRACT

Retention of original pore space, development of secondary pores, and precipitation of CaCO3 cement in pores of sedimentary reservoir rocks are the result of a delicate balance among organic processes, composition of water and migration of fluids, and the depositional and diagenetic history of calcium carbonate sediment particles and carbonate rocks. In a high-energy marine environment, where waves and currents impinge on the sediment particles, original pore space may persist and thus may be available in subsurface reservoirs. However, in reefs, cements are precipitated early. Original pore space will be preserved only if hydrocarbons are introduced early, if shale seals off the reef from migrating fluids, or if the reef is invaded by meteoric waters undersaturated with respect to CaCO3. Where reefs or ooid and skeletal accumulations are subject to leaching by meteoric water subaerially, in the subsurface, or on the sea bottom near leaks of meteoric water, leaching creates secondary (moldic) pore space while solutions are undersaturated with respect to CaCO3. These new secondary pore spaces are added to the reserved original pores; they increase the total porosity. In corals leaching begins in sclerodermites. The newly created pores merge and form channels. In such leached corals porosity values may exceed the porosity values found in corals of unleached modern reefs. Hence conditions for developing maximum pore space include exposure of reefs or high-energy carbonate sands to meteoric waters. However, once meteoric waters become saturated with respect to calcite, calcite cement is precipitated in the pores. This cement progressively eliminates primary and secondary pores. Under both surface and subsurface conditions, sulfate-reducing bacteria use sulfate from seawater, or from meteoric or formation waters as an oxidant for that part of organic matter which they oxidize for energy production. Calcite as a cement is formed when CO2 produced in the bacterial oxidation of organic matter combines with calcium. CaS, an intermediate product in sulfate reduction, hydrolizes, because it is not stable in aqueous solution, and in this reaction the pH rises. At high End_Page 1907------------------------------ values of pH not only does calcite cement tend to be precipitated, but also quartz particles tend to be peripherally corroded and ultimately replaced. Such cementation and replacement of quartz by calcite have been observed in subaerially exposed sandstones <10,000 years old, in subsurface reservoir rocks, and in modern beachrocks. Other bacteria form CO2 from methane, such as that found in lagoonal sediments or tidal marshes, and this CO2 can trigger precipitation of carbonate cement in sediments lying on the sea floor or occurring in the subsurface. End_of_Article - Last_Page 1908------------