Shallow Subtidal Environments Research Articles

A Scheme for Multivariate Analysis in Carbonate Petrology with an Example from the Silurian Tonoloway Limestone

ABSTRACT Two methods of exploratory data analysis--a numerical classification system applied in conjunction with an ordination technique--were used to facilitate environmental interpretation of 61 Tonoloway Limestone samples from Pinto, Maryland. We suggest that the scheme of multivariate analysis presented here, because it involves nonparametric methods, is more applicable to typical geologic data than other strategies. In the Tonoloway Limestone 7 facies (clusters) were identified by Q-mode cluster analysis: (1) micrite, (2) micrite-dolomite, and (3) dolomite microfacies from the supratidal zone; (4) a micrite-peloid microfacies of the intertidal mud flat; (5) intraclast and (6) ooid-calcite cement microfacies representing a shallow-subtidal environment; and (7) a peloid-calcite cement micro acies from the deeper-subtidal environment. Multidimensional scaling illustrated interrelationships among the clusters and the gradational nature of samples; neither factor was observed by cluster analysis alone. Recognition of the gradational nature of samples then led to interpretations of three environmental gradients that were normal to the paleoshoreline. Hydrodynamic energy was low on both the tidal flat and offshore near wave base and systematically increased from both directions toward the shallow-subtidal environment. Petrographic fossil diversity was highest near wave base with a second, lesser peak on the intertidal mud flat; this variable decreased into the shallow-subtidal high-energy belt and also onto the supratidal mud flat. Dolomitization, apparently of two modes, was mo t pronounced in rocks of the schizohaline supratidal zone.

Silurian trilobites from arctic Canada

SummaryTrilobites of upper Ludlow age are described from Member A of the Read Bay Formation on Cornwallis Island. Four new species are represented: Helokybe spio gen. et sp.nov. (Dechenellinae), Encrinurus hyperboreus sp. nov., Kailia? capra sp.nov. (Encrinurinae) and Hemiarges echinatus sp.nov. Other taxa present are: an effaced scutelluid; Cyphaspis sp.; a warburgelline; two more encrinurids; two other forms of Hemiarges. The trilobites occur in sediments indicative of a shallow subtidal environment, with the exception of the warburgelline which occurs in sediments deposited under deeper subtidal conditions below storm wave-base.

A trace fossil preserving its producer (Trentonia shegiriana) from the Trenton Limestone of the Quebec City area

The Trenton Limestone of the Quebec City area contains a moderately abundant and diverse ichnofauna including numerous simple trails that formed at the sediment–water interface. A rare specimen of one of these trails demonstrates that its producer was an errant polychaete annelid, Trentonia shegiriana gen. et sp. nov. It is suggested that T. shegiriana or a morphologically similar errant polychaete annelid was responsible for the production of not only the trails described herein but also the ichnospecies Walcottia rugose. Faunal, ichnofaunal and sedimentological evidence suggests that T. shegiriana inhabited a shallow water subtidal environment with water depths in the order of 10 m or less and preferred a calcisiltite substrate. In the absence of T. shegiriana these simple trails can therefore be utilized as useful palaeoenvironmental indicators.

Salina A-1 Sabkha Cycles and the Late Silurian Paleogeography of the Michigan Basin

ABSTRACT In the shelf belt surrounding the Michigan basin, the A-1 Unit at the base of the Upper Silurian Salina Group comprises in stratigraphic succession the A-0 Carbonate, the A-1 Anhydrite and the A- l Carbonate subunits, The unit consists of six lithofacies representing three depositional environments: microlaminated mudstone and pelletal wackestone deposited in a shallow subtidal environment; planar stromatolite deposited in an intertidal environment; nodular anhydrite and indurated dolomite crusts formed on supratidal fiats and intraformational breccia and flat pebble conglomerate, formed on supratidal and intertidal flats and occasionally distributed to the nearby subtidal zone. With few exceptions, all carbonate facies consist of dolomicrite, are completely devoid of skeletal remains contain early diagenetic nodular and acicular anhydrite and authigenic framboidal pyrite. With the overlying A-2 Anhydrite, the A-1 Unit records the superposition of four successive sabkha cycles. At the beginning of Salina time (late Wenlockian-early Ludlovian?) intensive evaporation caused the gradual withdrawal of the sea to the central part of the basin and the progressive exposure of the circumferential Guelph carbonate banks and pinnacle reefs. The basal part of the Salina Group in the basin center area was deposited in a water depth of about 800 ft. The evaporative withdrawal led to the near complete desiccation of the basin during which time potash salts were deposited. The subsequent shallow water evaporite deposits of the middle part of the Salina gradually filled the basin. These deposits successively onlapped the basin margins along a surface which represents a regional unconformity. The A-2 Carbonate was the first Salina unit which eventually transgressed the south rn margin. It is most probably correlative to the Greenfield Dolomite on the Indiana-Ohio platform. The Salina B, D and F Salts were also formed in shallow water. Their deposition was contingent upon synsedimentary basin subsidence. Probably most of this subsidence took place during the relatively long time spans represented by the abundant carbonate horizons within and between the main evaporite cycles. Abundance of organic matter of algal origin and the prevalence of reducing conditions soon after deposition make ancient sabkha carbonates interbedded within thick evaporite deposits potential source beds for hydrocarbons. The environmental interpretation of the A-1 Unit provides insight into the problem of the origin of evaporite-encased reefs. What appears to be contemporaneous deposition of incompatible reef and evaporite facies is best resolved when viewed as a three-stage evolution comprising a reef-building period, an evaporative withdrawal phase and an evaporite deposition period.

Pore Space Reduction in Cretaceous Sandstones through Chemical Precipitation of Clay Minerals

ABSTRACT Volcaniclastic sandstones from the Upper Cretaceous Horsethief Formation, Montana, are characterized by two distinct cement assemblages; (1) Corrensite ± Calcite ± Dolomite, which is restricted to samples from delta distributary channel and distributary mouth bar environments, and (2) Montmorillonite ± Calcite, which characterizes bay-beach, crevasse splay, lagoon, barrier island, and shallow subtidal environments. Petrographic, microprobe, scanning electron microscope, and fluid inclusion analyses indicate the early stage origin of these assemblages as authigenic chemical precipitates. In Horsethief volcaniclastics, pore-space reduction by precipitation of the clay minerals corrensite and montmorillonite reflects the chemistry of early stage pore-fluids, and does not involve syn- or post-depositional introduction of detrital clays.

Sedimentology of a Middle Ordovician quartz arenite–carbonate transition in the Upper Mississippi Valley

The St. Peter—Platteville transition consists of three formations representing an upward change in deposition from quartz arenites to shallow-water carbonate rocks. The St. Peter Sandstone is the lower unit and is composed of conglomerate and shale overlain by quartz arenites; it is divided into the Kress, Tonti, and Starved Rock Members. The Glenwood Formation is the middle unit and consists of argillaceous sandstone, arenaceous mudstone, fissile shale, and arenaceous dolosiltite. The Pecatonica Formation is the upper unit and consists of laminated and unlaminated carbonate rocks, sandstone with carbonate cement, and brecciated calcisiltite. The Kress and Tonti Members were deposited as basal units during transgression over an erosional surface, and the Tonti Member accumulated as a sheet sand in progressively shallower water northward. The transgression was followed by recession due to localized aggradation. The Starved Rock Member was deposited as a barrier-island complex, while the Glenwood Formation accumulated in a lagoon north of the barrier island. Transgression resumed, and the Pecatonica Formation was deposited in a shallow subtidal environment over most of the area. Deeper water carbonate rocks, however, formed where the former lagoon was deepest, and tidal flat carbonate rocks formed over the submerged barrier island.

Early Cambrian Shallow Subtidal Environment--Time and Place for Taxonomic Expansion: ABSTRACT

Early Cambrian Shallow Subtidal Environment--Time and Place for Taxonomic Expansion: ABSTRACT

Conceptual Model of Tidally Influenced Deposition on Margins of Epeiric Seas: Lower Ordovician (Canadian) of Eastern New York and Southwestern Vermont

Ten carbonate lithofacies are recognized in the Lower Ordovician (Canadian) Great Meadows and Fort Ann Formations in eastern New York and adjacent southwestern Vermont. These lithofacies developed on a shallow shelf adjacent to the deeper proto-Atlantic Ocean on the east, and were deposited within the tidally influenced zone along the margin (pericontinental zone) of a craton-wide epeiric sea. Shelf sedimentation involved the formation of a widespread carbonate tidal-flat complex, and was succeeded temporally by a lithofacies mosaic which reflected the balance between the laterally discontinuous progradation of low-energy carbonate tidal flats and algal shoals, and the patchy submergence by quiet-water, fossiliferous subtidal limestones. Shelf-marginal lithofacies appear to have consisted of high-energy oolite shoals which persisted throughout early Canadian time. Periodic circulation restriction between inner-shelf and open-ocean waters by marginal shoals and areal restriction of shallow subtidal environments by extensive prograding tidal flats affected salinity fluctuations within shelf waters, from normal to hypersaline conditions. Widespread intertidal environments at times isolated local evaporite-depositing salinas on desiccated tidal flats.

Facies distribution patterns of some marine benthic faunas in early paleozoic platform environments

Worldwide Late Cambrian—Silurian lithofacies patterns indicate that the platforms of that time were sites of accumulation of two essentially different rocks suites: the platform carbonate rocks and the platform terrigenous rocks. Most of the platform rocks accumulated as sediments in shallow marine environments similar to those of the present but far more widely spread. Present-day marine benthic faunas are distributed in depth zones which are primarily controlled by temperature. Faunas tend to occur in substrate-related discrete clusters (communities) within each life zone; similar substrates in different depth zones commonly have different faunal associations. Individual phyletic stocks may encounter environmental optimum or near-optimum conditions in certain areas, that commonly are revealed by an abundance of species and individuals within species in each stock. Environmental optimum conditions depend upon availability of food that may be utilized, modes of feeding of the animals present, water motion, and substrate, among other factors. Organisms in past seas were distributed in patterns similar to those of the present. Carbonate platforms were particularly widespread during the latest Cambrian—Early Ordovician. Intertidal environments spread widely across those platforms during that time and characteristic faunal associations developed in them. Saukiid and related tribolites dominated latest Cambrian carbonate platform intertidal faunas. The Early Ordovician carbonate platform intertidal was dominated by archeogastropod-nautiloid cephalopod faunas. These animals were joined by tabulate corals and certain brachiopods during the latter part of the Ordovician and Silurian as prominent faunal elements in the carbonate platform intertidal—shallow subtidal. Cruziana and related trace fossils, bivalves, and certain tribolites (notably homalonotids and dalmanitids) dominated most terrigenous platform intertidal—shallow subtidal faunas of the Ordovician and Silurian. Articulate brachiopods (primarily orthoids, strophomenoids, and rhynchonelloids) appear to have been relatively prominent during the Early Ordovician in shallow subtidal environments on both carbonate and terrigenous platforms and to have spread down the bathymetric gradient into increasingly deeper subtidal areas of both platforms during the latter part of the Ordovician. Tribolites dominated faunas in relatively moderate to deep subtidal environments on both platforms during the early part of the Ordovician. They were gradually replaced by brachiopods in first the shallower, and later the deeper subtidal as dominant members of the faunas. Brachiopods (primarily pentameroids and spiriferoids) dominated nearly all Silurian warm-water subtidal environments from the shallow subtidal to the edges of the platforms. Platform uplifts in the Middle Ordovician and glacio-eustatic sea-level fluctuations in the Late Ordovician caused environmental changes across the platforms that were accompanied by marked replacements among marine benthic faunas in all environments. The distribution of Ordovician carbonate platforms and glacial deposits suggests that an Ordovician polar region may have been close to present-day equatorial Africa and that Ordovician warm temperate-tropical regions lay close to the present-day North Pole.

Stratigraphy of Everton Formation (Early Medial Ordovician), Northern Arkansas

The Everton Formation is an intertonguing complex of sandstone, limestone, and dolomite exposed in northern Arkansas; it has produced gas shows in the subsurface. The Everton lithologies outcropping in bluffs along the west-east trending Buffalo and White Rivers can be divided into seven laterally persistent members. Previous investigators named 4 members but were unaware of their regional relations within the Everton. Three new informal members are defined. The basal unit, the Sneeds Dolomite Member, disconformably overlies the Powell Dolomite (Canadian) and is laterally persistent. Members A and B consist of interbedded sequences of limestone and sandstone above the Sneeds. In the east, member A underlies a massive St. Peter-like sandstone known as the Calico Rock Sandstone Member, and, in the west, member B underlies the thick Newton Sandstone Member. Unnamed Everton dolomites include a medium crystalline dolomite facies, a late secondary replacement of limestone, exposed in the central part of the traverse, and a very finely crystalline dolomite facies with supratidal characteristics in the central and eastern parts of the traverse. Member C is a laterally persistent dolomitic sandstone-dolomite complex overlying the Newton sandstone n the west and the unnamed Everton dolomites in the central and eastern parts of the study area. The uppermost member, the Jasper, is also extensive and undergoes an eastward lithologic change from sandstone to limestone to dolomite. The units of the Everton lack an abundant macrofauna except for the limestone facies of the Jasper Member. The meager fauna, lithologies, and vertical and lateral relations of the members record deposition on a shallow, broad shelf. The carbonate rocks accumulated in shallow subtidal, lagoonal, and supratidal environments, whereas the quartz sands were deposited in barrier-island, chenier-plain, and shallow-subtidal environments. The basinal area was on the south in the Ouachita geosyncline, and the intermittently exposed craton that acted as the source area for the terrigenous material lay on the north. As a result of this study, the Everton can be correlated with equivalent deposits exposed in northeastern Oklahoma (Burgen Sandstone and lower and middle Tyner Formation). Tentative correlation with the Simpson Group of central and southern Oklahoma also is proposed.

Interpretation of Carbonate-Detrital Silicate Transitions in the Carboniferous of Western Wyoming

A conceptual model of sedimentation in a complex of subtidal, tidal-flat, sabkha, and dune environments facilitates the interpretation of stratigraphic contacts between carbonate and detrital silicate rocks. The boundary between carbonate and detrital silicate sedimentation approximates the reach of the highest tides, and is a zone of transition between the predominantly carbonate coastal sabkha and the predominantly detrital silicate continental sabkha. With regressive sedimentation, the horizontal sequence of facies from marine carbonate sediment landward to the detrital silicates of the continental sabkha results in a stratigraphic sequence of carbonate rock conformably overlain by detrital silicate rock. The Lower Carboniferous strata in western Wyoming appear consistent with that model; thus, the contact between the carbonate strata of the Madison Group and the detrital silicate rock at the base of the Amsden Formation is conformable. The Lodgepole Formation is predominantly of shallow subtidal origin. The Mission Canyon Formation is the product of tidal-flat sedimentation. Regressive sedimentation of the Madison Group culminated with the creation of a land surface on which sabkhas and dune fields developed. The lower beds of the Amsden Formation are the continental sabkha and dune deposits. In the Upper Carboniferous, the middle part of the Amsden Formation records a period of time when an increased supply of terrestrial detritus shifted the carbonate-detrital silicate boundary from the high-tide line into the shallow subtidal environments. Carbonate tidal-flat deposits at the base of the upper part of the Amsden Formation indicate a return to the sabkha environment. The repeated interbedding of carbonate tidal-flat and sabkha deposits with detrital silicate sabkha and dune deposits throughout the upper Amsden and Tensleep Formations resulted from numerous local transgressions and regressions, during which the area never was far removed from the coastal-continental sabkha boundary.

Transition from Shelf to Basinal Carbonates in Middle Cambrian Marjum Formation of House Range, Western Utah--Paleoenvironmental Analysis: ABSTRACT

Lithofacies of the Marjum Formation in the House Range of west-central Utah indicate that a rather abrupt transition from shallow-shelf to basinal depositional environments persisted in this locality during much of the Middle Cambrian. The beginning of Marjum deposition was marked by the accumulation of deep-water limestones and shales throughout the area, but this pattern soon was disrupted by shoaling and development of shelf conditions in the vicinity of the central and northern House Range. Supratidal to shallow subtidal carbonates formed on the shelf as deposition of slope and basinal sediments continued a few miles south for the remainder of Marjum time. Algal-laminated and pelletal dolomitic mudstones accumulated in the extensive supratidal and intertidal areas that existed on the shelf. Some stromatolitic units are evenly laminated, whereas others have laminae that are irregular and discontinuous. The pelletal mudstones exhibit fenestral fabrics and contain intraformational conglomerate lenses, commonly including clasts reworked from the stromatolitic units. Wave ripples and desiccation features associated with these lithologies are further evidence of their shoal-water origin and periodic exposure. Pelleted, intraclastic wackestones and packstones characterize the shallow subtidal environments of the shelf and shelf-slope. Interspersed with the slope sediments are units containing both algal-coated and uncoated intraclasts. The coated grains are on the upper part of the slope and were derived from the shelf edge, whereas the uncoated clasts were largely reworked from slope deposits. In addition, the coated grains have undergone little transport, but the uncoated clasts are well sorted and rounded, and are commonly graded. Fossiliferous wackestones and laminated mudstones with interbedded shales represent the deeper water toe-of-slope and basinal sediments. The fossiliferous units have been moderately bioturbated, but the finely laminated mudstones, which were deposited well below wave base, are essentially undisturbed. The presence of sponge spicules and presence of small-scale cross laminations and microscopic cut-and-fill structures are typical of these dark, thin-bedded limestones. Depositional slopes in this part of the basin are indicated by penecontemporaneous slump structures and debris lenses deposited by submarine slides. End_of_Article - Last_Page 771------------

Significance of Thin Carbonates in Interpreting Depositional Environments of Thick Clastic Sequences: ABSTRACT

When interpreting depositional environments of dominantly clastic sequences, thin carbonates, if present, often are overlooked or given short shrift. Detailed study of the carbonates, however, can be instrumental in environmental interpretations of enclosing clastics. This is particularly true if the clastic units lack fossils or environmentally significant sedimentary structures. This hypothesis is supported by 2 examples from Pennsylvanian and Permian strata of southeastern Wyoming. The Permian Goose Egg Formation consists of thick, red siltstone and mudstone with interbedded thin, widespread carbonates. The clastic units have been interpreted by various workers as deep-water marine, shallow marine, deltaic, specialized marine, or continental deposits. Petrographic examination of the carbonates suggests that they were deposited in shallow subtidal, intertidal, and supratidal environments. The facies mosaic exhibited by the carbonates suggests that enclosing siltstone and mudstone were deposited in nonmarine environments. Festoon cross-stratified sandstone which characterizes the Casper Formation (Pennsylvanian-Permian) in the extreme southern Laramie basin has been interpreted as marine, subaerial, or fluvial in origin. Carbonate beds in the Casper Formation are thin, lenticular lithosomes of limited geographic extent. Petrologic studies of these limestones suggest that they were deposited in small lakes or ponds which periodically were emergent. The inferred environment of carbonate deposition supports a subaerial dune environment for the festoon cross-stratified clastics. End_of_Article - Last_Page 622------------

Stratigraphy and Potential Prospects of Devonian Reefs of New York: ABSTRACT

Reefs are found in the outcrop sections of several Lower and Middle Devonian units in New York State. The most prominent of these occurs in the Edgecliff Member of the Onondaga Limestone. The Onondaga Limestone was first described and named by James Hall of the New York Geological Survey in 1839. The present fourfold division of the Onondaga, in descending order, Seneca, Moorehouse, Nedrow, and Edgecliff, was proposed by Oliver in the early 1950s. The type section is located in Onondaga County, New York. In the subsurface, the uppermost Seneca Member is a massive limestone and can only by separated from the similar underlying Moorehouse Member by the presence of the Tioga Bentonite Bed, which gives a characteristic peak on the gamma ray log. The Seneca is absent in the central-southern part of New York, where a pronounced thinning of the Onondaga occurs. The Moorehouse is a massive cherty limestone and is also missing in the extreme central-southern part of New York in the previously mentioned area of thinning. The Nedrow is a shaly cherty limestone and is persistent throughout the state and in the area of thinning, except over known subsurface reefs in the underlying Edgecliff. The lowermost Onondaga member, the Edgecliff, is a coarse-grained light-gray to grayish-white biostromal limestone, present in an area from northeastern southwestward through central New York. In eastern and southeastern New York this unit is represented by an argillaceous facies, whereas in far western New York it is highly cherty. The Edgecliff shows a pronounced thinning in central and southern New York and in north-central Pennsylvania, where it is mostly 10 ft or less thick. In the southwestern part of this thin area, three subsurface Edgecliff reefs, all 150-200 ft thick and containing gas, have been discovered since 1967. At least 21 smaller reefs are known in the outcrop section of this member in eastern New York, one in central New York and two in the Buffalo area. The reefs were formed in a clear-water shallow subtidal environment on the Edgecliff platform. Biostromal facies and reefing are also present on the outcrop in several zones in the Middle Devonian Hamilton Formation, which overlies the Onondaga. Most important of these zones are in the Ludlowville Member of the Hamilton in the Syracuse area of central New York. Two of these zones, the Joshua and Staghorn Point, occur over areas of 40 and 120 sq mi, respectively, according to Oliver. No reef buildup in these zones has been encountered in drilling as yet, but no systematic search has been made for reefs in the subsurface. Several smaller reefs are known from outcrops of the Coeymans Formation of the Helderberg Group in central New York and northwestern New Jersey. End_of_Article - Last_Page 2110------------

Paleogeography and Depositional Environments of the Cambro-Ordovician Shallow-Marine Facies of the North Atlantic

Detailed sedimentological and petrological investigations of the Cambro-Ordovician shallow-marine facies of western Newfoundland, northwest Scotland, and central East Greenland reveal arresting similarities in their histories. Faunal, stratigraphic, sedimentologic, geochemical, and diagenetic comparisons of the thick sandstone, shale, and carbonate successions indicate contemporaneous development on a contiguous broad marine shelf bordering the western margin of a “proto-Atlantic” basin. Sedimentation dominated by intertidal to shallow subtidal environments is indicated by tidal sand bodies, ichnofossils, desiccation phenomena, oolitic lithologies, and stromatolitic algal structures. Complex diagenetic processes probably account for anomalous potash concentrations in correlative Lower Cambrian shale horizons and demonstrably effect replacements by feldspar, silica, dolomite, and calcite in the sandstones and carbonates. Parallel trends of recrystallization, dolomitization, silicification, calcitization, and dolomitization of the oolitic carbonates in each of the three study areas suggest correspondent post-depositional diagenetic environments. Independently the similarities are inconclusive, but collectively they are compelling in their suggestion of common depositional and diagenetic histories for the Cambro-Ordovician successions of western Newfoundland, northwest Scotland, and central East Greenland, ostensibly on the western shelf of a “proto-Atlantic” basin.

Algal Mudstone Mounds in Morrowan Stage (Lower Pennsylvanian) in Northeastern Oklahoma: ABSTRACT

The middle of the Morrowan Stage is marked by the development of an algal carbonate mudstone. The area of exposure covers 700 sq mi in northeastern Oklahoma with a maximum development of 60 ft in the southwestern 30 sq mi of the outcrop. A broad algal bank and smaller algal mounds developed within the area of maximum thickness, on the southwestern margin of the Ozark uplift. This area has high faunal diversity and the bank is cut irregularly by channels of skeletal sandstone. Northeastward, the faunal diversity of the unit is low, and the mudstone thins owing to replacement by skeletal grainstones and shale. The algal mounds are up to 6 ft high and 10 ft across. The core material consists of Archaeolithophylum and Cuneiphycus mudstone and boundstone, whereas the flank and intermound material consists of coarse skeletal packstone and wackestone. Influx of fine terrigenous clastics occurred during formation of this unit, as shale is present in small thin streaks and pockets. Locally, oolitic packstones and beds of algal oncoliths are found in the top. The mudstone is encompassed between skeletal grainstone throughout the areas of exposure. The overall dearth of skeletal debris, abundance of algae, occurrence of stromatolite-type boundstone, burrowing, and occurrence of dolomite indicate that the mudstone was formed in a shallow subtidal or tidal-flat environment. Abundant recrystallization of matrix mud to microspar and pseudospar has taken place, and dolomite, ferroan dolomite, and siderite are present locally as replacement of skeletal debris and mud matrix. End_of_Article - Last_Page 362------------

EARLY VOID‐FILLING CEMENTATION IN DEVONIAN FORE‐REEF LIMESTONES (GERMANY)

SUMMARYFibrous calcite is the main cement in the voids and cavities of Devonian fore‐reef limestones in Germany. Cementation occurs in some cases simultaneously with encrustation by blue‐green algae as well as internal deposition of calcarenites and calcilutites. These observations lead to the conclusion that the fore‐reef voids were exposed to light and were still in free communication with the sea, when the cementation began. Furthermore, redeposited fragments of former cavity fillings indicate that the formation of cavity‐filling fibrous calcium carbonate cement was already completed before the sea‐ward erosion took place. It is believed that the precipitation of fibrous calcium carbonate in most of the Devonian fore‐reef limestones, which show no signs of dissolution, occurred in a shallow subtidal environment. Only the voids showing larger solution phenomena were probably exposed to fresh water in the inter‐ to supratidal zone.

Bivalves as bathymetric indicators

Water depth does not directly control the distribution of marine bivalve molluscs, but bathymetric inferences may sometimes be made by relating water depth to environmental variables known to limit bivalve distribution, such as temperature, salinity, and nature of the substratum. When contemporaneous stenothermal and eurythermal (or stenohaline and euryhaline) bivalve associations are found in the same general area, the more tolerant associations (eurythermal or euryhaline) will usually be found in shallower water than less tolerant (stenothermal or stenohaline) associations. Similarly, associations dominated by epifaunal and boring bivalves would normally be expected in relatively shallow water where hard substrata are most abundant. Burrowing infaunal bivalves are present at all depths, but infaunal deposit-feeding bivalves are most common in subtidal, quiet water muds. Observations on Recent infaunal benthonic associations suggest that organisms adapted for deep vertical burrowing are common only in intertidal and shallow subtidal environments where stress conditions at the sediment-water interface are most severe. In contrast, infaunal organisms adapted for horizontal burrowing just below the sediment surface predominate in deeper subtidal environments. In situ associations of fossil bivalves adapted for these two modes of burrowing should therefore be reliable indicators of relative water depth. These burrowing patterns may also be determined directly from biogenic sedimentary structures, and thus this paleobathymetric tool may be applied to many unfossiliferous sedimentary rocks.

Recognition of Transgressive Carbonate Sequence Within Epeiric Sea: Helderberg Group (Lower Devonian) of New York State: ABSTRACT

The regional Late Silurian-Early Devonian marine transgression of the central Appalachians is represented in New York State by a shallow-water carbonate rock sequence (Helderberg Group) which locally transgressed north and west. The resultant stratigraphic section comprises several hundred feet of fossiliferous limestone which has several distinctive sedimentary facies. Early workers interpreted each of the major facies as a separate time-stratigraphic lithologic unit or formation. However, from detailed field examination Rickard (1962) demonstrated that these formations are in fact time-transgressive toward the west and interfinger laterally with each other. Paleoecological study of the Helderberg Group supports this interpretation and shows that each of the formations represents a local sub-environment within the transgressive interval as a whole. These formations (facies) are: (1) Manlius Formation (25-50 feet), a complex of rock types interpreted to represent supratidal, intertidal, and shallow subtidal environments within a broad shelf lagoon (Laporte, 1964; 1967). (2) Coeymans Formation (20-100 feet), crinoidal-brachiopod skeletal calcarenite and carbonate siltstone which are commonly burrow-mottled toward the base of the unit but which show increasingly greater evidence of current reworking toward the top (high-and low-angle cross-stratification and sheet deposits). The Coeymans is interpreted to have been deposited in a wide belt of shallow, submerged crinoid mounds and banks which served as an effective, though discontinuous, barrier to circulation separating the more open-marine environment on the east from the restricted shelf lagoon of the Manlius on the west (Anderson, 1965). (3) Kalkberg Formation (50-100 feet), highly burrow-mottled carbonate mudstone with a very abundant, diverse, and well-preserved biota. The Kalkberg is interpreted to be a shallow-water, open-marine deposit which developed on an extensive shelf seaward from the Coeymans crinoid banks and meadows. (4) New Scotland Formation (50-150 feet), highly argillaceous and siliceous carbonate mudstone with a somewhat less diverse and abundant biota than the Kalkberg. The New Scotland is interpreted as having developed on a broad shelf like the Kalkberg (and marginal to it), but with significantly greater influx of terrigenous detritus which probably came from a distant easterly source. Lateral and vertical variations in constituent carbonate-grain types, mudstone-sparite ratios, fossil abundance and diversity, and presence of primary sedimentary structures provide criteria for recognizing the transgressive nature of the major sedimentary facies of the Helderberg Group. The inferred depositional framework, moreover, is very similar to that postulated by Shaw (1964) and Irwin (1965) for clear water sedimentation within an epeiric sea and demonstrates the predictive validity of their generalized sedimentary model. End_of_Article - Last_Page 473------------

Biogenic Graded Bedding

ABSTRACT Graded beds may be produced by deposit-feeding organisms in intertidal and shallow subtidal environments. Barnstable Harbor, Cape Cod, Massachusetts, contains intertidal populations of Clymenella torquata, a sedentary polychaete. Clymenella selectively ingests specific size fractions of sediment at a depth of 10-30 cm within the tidal flats. Sand and silt are transported vertically through the gut and egested as uncompacted feces on the sediment surface. Size fractions too coarse for ingestion ( > 1.0 mm) are concentrated at the lower end of worm tubes producing a well graded stratum. Laboratory measurements indicate that Clymenella is capable of displacing 87 liters of sediment per m2 per year at the mean annual water temperature of Barnstable Harbo (11°C). The thickness of a graded bed, limited by feeding depth, is less than 30 cm. Maximum and average grain size decrease in regular fashion from the base to the top of the bed. Sorting is poor and values vary irregularly in the bed. The size distribution curves are strongly to moderately fine-skewed in samples at the base of beds; curves become more nearly symmetrical at the top of graded beds. Biogenically graded beds may be distinguished from physically deposited graded beds by textural properties, associated sedimentary structures, geometry of the deposits, and lithology of interfingering facies. The best development of biogenic graded bedding occurs in sites of slow hysical sediment accretion and relatively weak bottom currents.