Effective Wave Base Research Articles

The earliest <i>Ancistrolepis</i> (Gastropoda: Buccinidae) and its geologic implications

The buccinid gastropod Ancistrolepis carolineae Squires, 1984 is the earliest known and shallowestmarine species belonging to the extant genus Ancistrolepis Dall, 1895, which is endemic to the NorthPacific region. This species is of late early Eocene (Ypresian Stage) age and is about 7 million years older than the previously oldest known record (late Eocene) of the genus. This rare species occursat several localities in a 1-m thick bed (“Stewart bed”) of fossiliferous shallow-marine sandstone within the Llajas Formation, on the north side of Simi Valley, Ventura County, southern California.The “Stewart bed” contains a rich fauna of subtropical mollusks and other invertebrates, which lived just below effective wave base, at the distal edge of a braid delta, immediately adjacent to an upper bathyal prodelta/slope environment, where a rich microfauna of calcareous nannofossils and benthic foraminifers lived. Dispersal of A. carolineae would have been either via drifting of buoyant “pouches” containing its non-planktonic eggs or by rafting of its adults in buoyant plant material debris. The dispersal of A. carolineae northward coincided with the cooling of the NorthPacific waters during the late Eocene, thereby providing the opportunity for Ancistrolepis to adapt to living in bathyal waters and also to living in association, in some cases, with chemosynthetic (methane) cold seeps.

Unfathomable: The shifting sand of wave base

ABSTRACTNotions of fair-weather, storm, and swell wave bases are ubiquitous in interpretations of wave-dominated siliciclastic shelves, carbonate ramps, and mixed-systems deposits that are present throughout the geologic record. A review of literature, observations of several modern and ancient depositional systems, and numerical hydrodynamic models reveal the roles of sediment grade, bathymetric irregularities, and depositional gradient on the variable depths to which waves leave a sedimentologically discernible record. They reveal that wave-induced horizontal particle velocities and estimates of wave effectiveness form a continuous spectrum with depth, with a lack of distinct subdivision into fair-weather and storm conditions. Although commonly ascribed to “fair-weather wave base,” the depth above which sediment is persistently agitated or winnowed also is shaped by bathymetric gradient, direction of wave approach, tides, and currents, as well as sediment grade. Similarly, even with identical waves, the maximum depth of initiation of sediment movement, e.g., effective wave base, is not directly comparable among shelves, or even within the same shelf through time, because hydrodynamic processes are encoded differently by shelves of variable morphology and sedimentology.The numerical models further suggest possible geomorphology–hydrodynamics–sediment linkages. Relative to steeper shelves impacted by identical waves, shelves of shallower gradient favor lower-energy seafloor conditions, likely accompanied by accumulation of finer or muddier sediment, thinner sandy shoreface accumulations, or both. Given that many stratigraphic accumulations from the parasequence to composite-sequence scale steepen upward with time, this conceptual model predicts an apparent increase in wave energy on the seafloor through time for individual progradational shorelines, even with constant waves.

Control of depositional and diagenetic processes on the reservoir properties of the Mishrif Formation in the AD oilfield, Central Mesopotamian Basin, Iraq

The results of a multiproxy study on Mishrif carbonates from the margins of the Central Mesopotamian Basin of Iraq display a wide range of rock fabrics and diagenetic features, all affecting reservoir quality and flow properties in a complex manner. Based on petrographic and facies analyses of the Mishrif Formation in the AD oilfield, the model of a homoclinal ramp geometry with twelve characteristic microfacies types is proposed. A relevant observation is that the ramp, judging from the core data of the AD oilfield studied here, reflects low-energy conditions as documented by an abundancy of fine-grained carbonates in nearly all microfacies types. This notion is perhaps best understood in the context of an intrashelf basin with significant wave-seafloor interaction leading to wave degradation and a shallow effective wave base. In contrast to the rudist bioherms that typify the more basin-marginal settings, we find mainly evidence for small, low-relief buildups characterized by green algae. The Mishrif reservoir properties are shaped by two main diagenetic features: (i) early diagenetic, aquifer-related marine-meteoric mixing zone dissolution inducing abundant fabric selective porosity and (ii) the preferential cementation of Thalassinoides burrows leading to low-porosity concretionary carbonate nodules with reservoir properties that differ significantly from those of the host carbonate. Given the somewhat more distal position of the AD oilfield, the effects of prolonged stages of subaerial exposure related to sea-level fall, as reported from more basin marginal settings, are not recognized. The bending and splaying of seams around nodules and the well-preserved skeletal material within these concretionary carbonates demonstrate that the onset of nodule cementation was early diagenetic/pre-compactional in nature. Based on integrated microfacies and reservoir properties, seven rock types (i.e., Rock Type I-VII) with distinctive reservoir qualities are recognized. Nodular concretions are mainly developed in the Rock Type I lithology and result in distinctive cm-to dm-scale heterogeneity. The contrast between small, poorly-connected pore systems in nodules and medium-sized, well-connected pore systems in the host rock are best described as an inconspicuous, dual permeability system. Data shown here and evidence from other studies dealing with the Mishrif in the Central Mesopotamian Basin are important to understand and predict reservoir properties in other carbonate fields with similar properties.

Ramp sedimentation across a middle Albian, Arctic embayment: Influence of subsidence, eustasy and sediment supply on stratal architecture and facies distribution, Lower Cretaceous, Western Canada Foreland Basin

AbstractRegional mapping of Middle Albian, shallow‐marine clastic strata over ca 100 000 km2 of the Western Canada Foreland Basin was undertaken to investigate the relationship between large‐scale stratal architecture and lithology. Results suggest that, over ca 5 Myr, stratal geometry and facies were dynamically linked to tectonic activity in the adjacent Cordillera. Higher frequency modulation of accommodation is most reasonably ascribed to eustasy. The Harmon and Cadotte alloformations were deposited at the southern end of an embayment of the Arctic Ocean. The Harmon alloformation, forming the lower part of the succession, constitutes a wedge of marine mudstone that thickens westward over 400 km from <5 m near the forebulge to >150 m in the foredeep. Constituent allomembers are also wedge‐shaped but lack distinct clinothems, a rollover point or downlapping geometry. Ubiquitous wave ripples indicate that the sea floor lay above storm wave base. Deposition took place on an extremely low‐gradient ramp, where accommodation was limited by effective wave base. Lobate, river‐dominated deltas fringed the southern margin of the basin. The largest deltas are stacked in the same area, suggesting protracted stability of the feeder river. A buried palaeo‐valley on the underlying sub‐Cretaceous unconformity may have influenced compaction and controlled river location for ca 3 Myr. Adjacent to the western Cordillera, a predominantly mudstone succession is interbedded with abundant storm beds of very fine‐grained sandstone and siltstone that reflect supply from the adjacent orogen. Bioturbation indices in the Harmon alloformation range from zero to six which reflects the influence of stressors related to river‐mouth proximity. Harmon alloformation mudstone grades abruptly upward into marine sandstone and conglomerate of the overlying Cadotte alloformation. The Cadotte is composed of three allomembers ‘CA’ to ‘CC’, that represent the deposits of prograding strandplains 200 × 300 km in extent. Allomembers ‘CA’ and ‘CB’ are strongly sandstone‐dominated, whereas allomember ‘CC’ contains abundant conglomerate in the west. The dominantly aggradational wedge of Harmon alloformation mudstone records flexural subsidence driven by active thickening in the adjacent orogen: the high accommodation rate trapped coarser clastic detritus close to the basin margin. In contrast, the tabular, highly progradational sandstone and conglomerate bodies of the Cadotte alloformation record a low subsidence rate, implying tectonic quiescence in the adjacent orogen. Erosional unloading of the orogen through Cadotte time steepened rivers to the extent that they delivered gravel to the shore. These observations support an ‘anti‐tectonic’ model of gravel supply proposed previously for the United States portion of the Cretaceous foreland basin. Because Cadotte allomembers do not thicken appreciably into the foredeep, accommodation changes that controlled these transgressive–regressive successions were probably of eustatic origin.

Mud dispersal across a Cretaceous prodelta: Storm‐generated, wave‐enhanced sediment gravity flows inferred from mudstone microtexture and microfacies

AbstractDetermining sediment transport direction in ancient mudrocks is difficult. In order to determine both process and direction of mud transport, a portion of a well‐mapped Cretaceous delta system was studied. Oriented samples from outcrop represent prodelta environments from ca 10 to 120 km offshore. Oriented thin sections of mudstone, cut in three planes, allowed bed microstructure and palaeoflow directions to be determined. Clay mineral platelets are packaged in equant, face‐face aggregates 2 to 5 μm in diameter that have a random orientation; these aggregates may have formed through flocculation in fluid mud. Cohesive mud was eroded by storms to make intraclastic aggregates 5 to 20 μm in diameter. Mudstone beds are millimetre‐scale, and four microfacies are recognized: Well‐sorted siltstone forms millimetre‐scale combined‐flow ripples overlying scoured surfaces; deposition was from turbulent combined flow. Silt‐streaked claystone comprises parallel, sub‐millimetre laminae of siliceous silt and clay aggregates sorted by shear in the boundary layer beneath a wave‐supported gravity flow of fluid mud. Silty claystone comprises fine siliceous silt grains floating in a matrix of clay and was deposited by vertical settling as fluid mud gelled under minimal current shear. Homogeneous clay‐rich mudstone has little silt and may represent late‐stage settling of fluid mud, or settling from wave‐dissipated fluid mud. It is difficult or impossible to correlate millimetre‐scale beds between thin sections from the same sample, spaced only ca 20 mm apart, due to lateral facies change and localized scour and fill. Combined‐flow ripples in siltstone show strong preferred migration directly down the regional prodelta slope, estimated at ca 1 : 1000. Ripple migration was effected by drag exerted by an overlying layer of downslope‐flowing, wave‐supported fluid mud. In the upper part of the studied section, centimetre‐scale interbeds of very fine to fine‐grained sandstone show wave ripple crests trending shore normal, whereas combined‐flow ripples migrated obliquely alongshore and offshore. Storm winds blowing from the north‐east drove shore‐oblique geostrophic sand transport whereas simultaneously, wave‐supported flows of fluid mud travelled downslope under the influence of gravity. Effective wave base for sand, estimated at ca 40 m, intersected the prodelta surface ca 80 km offshore whereas wave base for mud was at ca 70 m and lay ca 120 km offshore. Small‐scale bioturbation of mud beds co‐occurs with interbedded sandstone but stratigraphically lower, sand‐free mudstone has few or no signs of benthic fauna. It is likely that a combination of soupground substrate, frequent storm emplacement of fluid mud, low nutrient availability and possibly reduced bottom‐water oxygen content collectively inhibited benthic fauna in the distal prodelta.

Long-Term Changes In Morphology and Textural Sediment Characteristics In Response To Energy Variation On Shoreface-Connected Ridges Off the East Frisian Barrier-Island Coast, Southern North Sea

Abstract Patterns of surficial sediment distribution across shoreface-connected sand ridges located along the barrier-island coast of the southern North Sea generally show a strong affinity between textural parameters and ridge morphology, comparable to that observed in other parts of the world. Thus, the sediment of the upper shoreface and the inner ridge crest consists of fine sand (water depths 12 m). In order to assess the mobility of the sediments and the consistency of the observed textural patterns, two high-resolution multi-beam bathymetric surveys were carried out along selected corridors across a ridge and trough system off Spiekeroog island, located along the southern North Sea coast of Germany, one in 2003, the other in 2007. The bathymetric data of the two surveys show only small-scale topographic changes. These were produced mainly by local changes in the bedform patterns on the inner ridge. The bathymetric surveys therefore document a remarkable morphological stability of the ridges over time periods of several years. Spatial and temporal variations in the distribution of standard textural parameters were assessed on the basis of 95 sediment samples recovered on the same grid in 1989 and 2005. These data show that, for example, the distribution of coarse sand (0–1 phi) became enhanced along the axis of the outer trough, while expanding onto the lower ridge flanks. Current measurements in fair weather demonstrate that the coarse sediments cannot be moved by the local tidal currents. The observed changes in the distribution of coarse sand, and in particular its expansion onto the adjacent ridge flanks, must therefore be associated with wave action, probably in combination with tidal currents. This is supported by an assessment of the entrainment potential of coarse sand (D = 1 mm) by typical waves during storms (T = 8 s; H = 2.5 m) and fair weather (T = 4 s; H = 0.7 m). The results demonstrate that the effective wave base for coarse sand during fair-weather conditions is limited to a water depth of 4.8 m, whereas during storms it exceeds a depth of 26.4 m. The mobility of the coarse sands in the outer trough at water depths > 16 m, and with it the morphodynamics of the ridges as a whole, is thus clearly controlled by the superimposition of storm waves on the tidal current regime.

Clinoforms, Paleobathymetry, and Mud Dispersal Across the Western Canada Cretaceous Foreland Basin: Evidence from the Cenomanian Dunvegan Formation and Contiguous Strata

Large-scale muddy clinoform stratification is rare in Cretaceous marine strata of the Western Canada Foreland Basin; most offshore mudstone units show near-parallel to gently diverging stratification that reflects differential basin subsidence. The early to middle Cenomanian Dunvegan Formation, of broadly deltaic character, is unusual in that it comprises a spectacular set of clinothems. These clinothems have been mapped over a 950 km dip transect, and downlap from NW to SE onto a phosphatic condensed section termed the FSU marker. Our regional mapping shows that the FSU marker is of basin-wide extent and marks the abrupt cessation of deposition for up to about 2 My in the south. In the northwest, the Dunvegan Formation forms a prominent wedge over 300 m thick, interpreted to fill a high-accommodation area. Muddy clinothems in this high-accommodation area are relatively tall (~ 80 m; 115 m decompacted) and extend about 80 km from the delta front to a zero edge at the downlap surface. Wave-rippled and hummocky cross-stratified fine sandstones of the upper delta front record the effects of storms, and extend about 30 m (~ 42 m decompacted) below contemporaneous beach facies, suggesting that effective wave base for sand transport was at about 40 m water depth; mud was transported in deeper water, presumably by storm-driven flows. Towards the SE, the Dunvegan Formation gradually changes from a wedge to a tabular shape, interpreted to indicate deposition in a low-accommodation area. In the transition from high- to low-accommodation areas, clinothems become progressively less tall (50 to about 20 m; 70 to about 29 m decompacted) but longer (150 to > 250 km). The most down-dip “clinothems” are essentially tabular and > 400 km in extent. In the high-accommodation area, offshore mud dispersal is inferred to have been limited by episodic geostrophic flows, and possibly by permanent along-shelf flows that swept mud (probably suspended, flocculated, and pelleted) into a shore-parallel prism, producing relatively short, steep clinothems. Muddy clinothems downlap onto sandy condensed facies of the FSU marker, the latter interpreted to have been deposited in shallow water prior to Dunvegan deposition, but which subsided below wave base in response to loading by the advancing deltaic clinothems. In the southern, low-accommodation area, tabular Dunvegan mudstones suggest that the sea floor was able to aggrade to effective wave base (i.e., about 40 m or less), at which point, wave resuspension of mud became frequent, resulting in efficient off- and along-shore dispersal of mud by geostrophic currents. The initiation of clinothem deposition is interpreted to coincide (a) with a phase of accelerated flexural subsidence in the northwest, related to renewed tectonic loading, and (b) possibly with the onset of large-scale anticlockwise marine circulation at the inception of the Greenhorn Seaway in the early Cenomanian. Together, these tectonic and oceanographic events formed an effective sediment trap along the western margin of the seaway. Only after the western sediment trap had filled, in late Dunvegan time, was mud again dispersed widely across the basin by storm processes in water less than about 40 m deep.

A GIS-Based Method to Determine Maximum Fetch Applied to the North Sea–Baltic Sea Transition

Abstract A GIS-based method that uses the visibility principle to calculate maximum fetch is presented. The software was programmed in the Arc macro language and implemented with the use of the GRID-based functions of the ArcInfo Workstation GIS. Applying L/4 as the effective wavebase and existing wind statistics, use of the method is exemplified by calculating the yearly resuspension frequency in the North Sea–Baltic Sea transition area.

FACIES DISTRIBUTION OF FUSULINIDA IN A BASHKIRIAN-MOSCOVIAN (PENNSYLVANIAN) CARBONATE PLATFORM TOP (CANTABRIAN MOUNTAINS, NW SPAIN)

The Sierra del Cuera (Cantabrian Mountains, Asturias, Spain) exhibits an exceptionally well-preserved upper Bashkirian-lower Moscovian succession of a high-relief carbonate platform with steep adjacent slope. During the late Bashkirian, the platform margin prograded rapidly basinward, and during the early Moscovian, platform aggradation increased. Fusulinida distributions have been estimated in three platform study windows representative of the Bashkirian and Moscovian lithofacies. Microbial boundstone accumulations at the platform margin and slope contain scarce fusulinoideans (particularly during the Moscovian aggradation, when water depth was relatively greater). Boundstones are characterized by tetrataxids, lasiodiscids, pseudoammodiscids, biseriamminids, endothyrids and palaeotextulariids. Grainstone units with sediments deposited above wave base are characterized by the predominance of Pseudoendothyra , Eostaffella (Bashkirian) or Profusulinella (Moscovian), Bradyina and palaeotextulariids. The open-marine facies, below effective wave-base but intermittently affected by storm currents, is enriched in endothyrids and palaeotextulariids in the Bashkirian part, and in Profusulinella , Eofusulina and Schubertella in the Moscovian part. The Moscovian facies, of lower energy and deeper water depth, contain some microbially precipitated micrite and have scarce Fusulinida, mostly represented by endothyrids, biseriamminids, and fusulinoideans ( Eostaffella , Ozawainella and Profusulinella ). Lagoonal open- to restricted-marine facies with beresellid algae contain Profusulinella , Schubertella and biseriamminids. Crinoidal packstones were deposited in moderate-energy settings close to the platform margin, below wave-base, and are characterized by Ozawainella , Eostaffella , palaeotextulariids and tetrataxids. These analyses show that fusulinid foraminifera inhabited mostly shallow and open-marine environments. Their distribution was controlled by depth-related variables, water energy and open-marine vs. restricted-marine conditions. Fusulinoideans increase in the Moscovian strata, and show different trends in the upper Bashkirian and lower Moscovian platforms, possibly because of: a) changes in the paleoecology of Fusulinida at the species level from the Bashkirian to the Moscovian; and/or b) variations in the depositional environments and physicochemical characteristics of seawater from the Bashkirian progradational phase to the Moscovian aggradational phase. Pseudoendothyra , Profusulinella , Schubertella , bradyinids and palaeotextulariids showed an affinity for high-energy settings. Profusulinella and Schubertella , however, preferred the shallowest facies and also tolerated restricted marine conditions. Eostaffella ’s distribution largely differs between the Bashkirian and Moscovian parts because it characterizes the high-energy grainstones of the upper Bashkirian and the open-marine, moderate- to low-energy environments of the lower Moscovian. Endothyrids were excluded from lagoonal environments with restricted circulation and abnormal salinity. Biseriamminids preferred open-marine, moderate- to low-energy settings with muddy substrates, but were not limited by water restriction. Greater tolerance to deeper, darker habitats was exhibited by lasiodiscids, pseudoammodiscids and tetrataxids, which occurred from the platform top down to 200 m depth on the Moscovian slope.

Shallow-water stratigraphy at the Mount Chalmers volcanic-hosted massive sulfide deposit, Queensland, Australia

Mapping of the Permian stratigraphy at the Mount Chalmers massive sulfide + footwall stockwork Cu-Au orebody has revealed shallow-water body fossils, trace fossils, and nonwelded to partly welded pyroclastic units within the fiat-lying host sequence. The massive sulfide-bearing Lower Chalmers sediment and the Upper Chalmers sediment are each 20 to 40 m thick and comprise fossiliferous tuffaceous mudstone and wacke. They are separated by the North Star pumice breccia. The breccia is 50 m thick at the mine and comprises watertransformed, pyroclastic-fiow deposits, but 2 km to the south and southwest it is 120 m thick and includes probable nonwelded ignimbrite. The Upper Chalmers sediment is overlain by a dacitic tuff which has a 5-m-thick, blocky to columnar-jointed interval near the base termed the Chalmers welded tuff. The tuff contains strongly spherulitic pumice lenses within a weakly spherulitic matrix with relict fine banding; it extends at least 450 m north and 300 m west from the mine. The Upper Chalmers sediment has yielded an assemblage of brachiopods, gastropods, bivalves, and bryozoans from five localities 1 to 2 km north and south of the mine. These body fossils show limited reworking and are typical of a shallow-marine shelf setting, near or below wave base. Although seawater depths of 50 to 300 m are possible, a range of 100- to 200-m depth is considered most likely. The Upper and Lower Chalmers sediments contain trace fossils typical of the Cruziana ichnofacies below and above the ore position, also suggesting shelf sedimentation below the effective wave base. These consistent estimates suggest that seawater depths of 200 to 300 m prevailed during deposition of the massive sulfide, contrary to the 1,000-m seawater depth usually regarded as necessary to form VHMS ore deposits. The shallow depth would have allowed the ascending ore-forming fluids to boil prior to reaching the sea floor, unless prevented by channel constrictions. Boiling may account for the strong, Cu-rich, footwall stringer zone and elevated fluid inclusion salinities at Mount Chalmers (Large and Both, 1980); the duplication of these features at other deposits suggests that Mount Chalmers is not an isolated case. The relatively shallow-water depths have also

Holocene paleoclimatic evidence and sedimentation rates from a core in southwestern Lake Michigan

Preliminary results of a multidisciplinary study of cores in southwestern Lake Michigan suggest that the materials in these cores can be interpreted in terms of both isostatically and climatically induced changes in lake level. Ostracodes and mollusks are well preserved in the Holocene sediments, and they provide paleolimnologic and paleoclimatic data, as well as biogenic carbonate for stable-isotope studies and radiocarbon dating. Pollen and diatom preservation in the cores is poor, which prevents comparison with regional vegetation records. New accelerator-mass spectrometer 14C ages, from both carbon and carbonate fractions, provide basin-wide correlations and appear to resolve the longstanding problem of anomalously old ages that result from detrital organic matter in Great Lakes sediments. Several cores contain a distinct unconformity associated with the abrupt fall in lake level that occurred about 10.3 ka when the isostatically depressed North Bay outlet was uncovered by the retreating Laurentide Ice Sheet. Below the unconformity, ostracode assemblages imply deep, cold water with very low total dissolved solids (TDS), and bivalves have δ 18O (PDB) values as light as — 10 per mil. Samples from just above the unconformity contain littoral to sublittoral ostracode species that imply warmer, higher-TDS (though still dilute) water than that inferred below the unconformity. Above this zone, another interval with δ 18O values more negative than — 10 occurs. The isotopic data suggest that two influxes of cold, isotopically light meltwater from Laurentide ice entered the lake, one shortly before 10.3 ka and the other about 9 ka. These influxes were separated by a period during which the lake was warmer, shallower, but still very low in dissolved solids. One or both of the meltwater influxes may be related to discharge from Lake Agassiz into the Great Lakes. Sedimentation rates appear to have been constant from about 10 ka to 5 ka. Bivalve shells formed between about 8 and 5 ka have δ 18O values that range from-2.3 to-3.3 per mil and appear to decrease toward the end of the interval. The ostracode assemblages and the stable isotopes suggest changes that are climatically controlled, including fluctuating water levels and increasing dissolved solids, although the water remained relatively dilute (TDS < 300 mg/l). A dramatic decrease in sedimentation rates occurred at about 5 ka, about the time of the peak of the Nippissing high lake stage. This decrease in sedimentation rate may be associated with a large increase in effective wave base as the lake approached its present size and fetch. A dramatic reduction in ostracode and mollusk abundances during the late Holocene is probably due to this decrease in sedimentation rates, which would result in increased carbonate dissolution. Ostracode productivity may also have declined due to a reduction in bottom-water oxygen caused by increased epilimnion algal productivity.

Depositional Environments and Porosity Development, Strawn Formation (Middle Pennsylvanian), Wagon Wheel and HSA (Penn) Fields, West Texas: ABSTRACT

The Wagon Wheel and HSA (Penn) fields are located on the western flank of the Central Basin platform within the Permian basin. In this area, both siliciclastic and carbonate depositional systems controlled sedimentation during Strawn time. To the east (HSA Penn field) siliciclastic fanglomerate sedimentation dominated. Several fanglomerate lobes, sourced from the Sand Hills uplift to the east, prograded westward across the platform. Interlobe swamps were common along this strand-line setting. Approximately 3 mi westward (Wagon Wheel Penn field), carbonate bank deposition dominated. Siliciclastic sands and clays commonly are intermixed within these carbonates and represent distal reaches of the siliciclastic progradational lobes. Conglomerates occur more rarely within the carbonate sequence. Carbonates from the upper Strawn (HSA 1392-P well, Wagon Wheel Penn field) demonstrate five shallowing-upward sequences. Each sequence consists of a lower subtidal phylloid algal wackestone which shallows upward into a grainstone shoal deposited above effective wave base. Transgression was rapid and marked by the subtidal phylloid algal wackestone facies overlying the grainstone shoal facies at the top of the previous sequence. Where present, chaetetids punctuate the contact between sequences. Porosity occurs as interparticle voids within the nearshore fanglomerate lobes and as leached biomolds in the grainstone shoals of themore » carbonate bank. Phylloid algal-rich facies are tight. Porosity is adversely affected by clastic pulses within the grainstone shoal facies and by late-stage ferroan calcite cement.« less

Stratigraphy of Transgressive Barrier Island Arc: ABSTRACT

Isles Dernieres, a Holocene transgressive barrier island arc on the Louisiana coast formed as a result of dominant marine processes reworking deposits of the abandoned Caillou distributaries of the early Lafourche delta lobe. The island system exhibits a variable transgressive and regressive stratigraphy throughout its framework, which is discriminate from comparable Atlantic coast transgressive barrier sequences. Continuous subsidence and storm breaching divide the island system into 3 major island segments: Western, Central, and Eastern Isles Dernieres, each of which is separated by locally interjacent tidal inlets. Vibracore data reveal that Western Isles Dernieres represents a series of accreting sand spits that resulted from the erosion and longshore transport of the abandoned distributary channel deposits from the central deltaic headland. This 3 to 4-m (10 to 13-ft) thick sequence of sand spits and associated marsh deposits overlies interdistributary silts and clays. Central Isles Dernieres is a fluvial-deltaic complex that exhibits marsh and tidal-flat deposits capping a relatively thick sequence of levee and interdistributary bay sediments. A relatively thin beach-ridge plain lies locally submerged beneath sand spit and marsh deposits of Eastern Isles Dernieres. This beach-ridge plain formed during a regressive phase of island evolution. Erosion of the beach-ridge plain above the effective wave base presently provides an active sand source for the downdrift accreting sand spit at the eastern end of Isles Dernieres. A high preservation potential of the western and eastern transgressive sequences of Isles Dernieres is probably due to rapid subsidence and the consequent in-place drowning of the island segments. In contrast, Atlantic coast barrier counterparts often exhibit incomplete transgressive sequences owing to continued shoreface erosion. End_of_Article - Last_Page 511------------

Late Paleozoic Foraminifera as Depth Indicators: ABSTRACT

Many late Paleozoic foraminifera consistently occur in particular rock types for which depths of deposition may be inferred. A few genera appear randomly in many different rock types and were possibly pelagic. Textulariina: In the Ammodiscacea, most genera of Hippocrepininae (Astrorhizidae) and Ammodiscidae are widespread and End_Page 542------------------------------ common in dark gray shales, siltstones, fine to very fine sandstones, and silty, calcareous mudstones deposited in basinal, slope, and deeper shelf setting below effective wave base. In the Lituolacea, the Hormosinidae, Lituolidae, Textulariidae, Trochamminidae, and Ataxophragmiidae include only a few late Paleozoic genera, which locally are abundant in dark to medium gray, silty shales, siltstones, silty sandstones, and carbonate wackestones and packstones; all deposited near wave base. Miliolina: Late Paleozoic representatives are primitive (tubular, nonseptate) genera of Hemigordiopsidae, Fischerinidae, and Nubeculariidae. These occur mainly in shallow, warm water calcareous wackestones. Fusulinina: These were the most taxonomically diverse of the late Paleozoic foraminifera and were adapted to a wide range of depth habitats. Parathuramminacea locally were very abundant in deeper water, dark gray calcareous wackestones formed in basins and slopes below wave base. Endothyracea (s.l.) included many genera and families that dominated most of the Early Carboniferous shallow water, calcareous depositional environments. Nodosinellidae preferred open shelf facies and may have extended to depths below wave base. Colaniellidae, Ptychocladiidae, Paleotextulariidae, Tetrataxidae, Tournayellidae, Endothyridae, Loeblichiidae, and Lasiodiscidae locally were common in shallow shelf, shoal, and lagoonal carbonate wackestones, packstones, and as displaced fossils in some grainstones, uch as oolites. Bradyinidae were globose, had pseudoalveolar walls, and were widely scattered in a number of different lithologies suggesting a pelagic or planktonic habitat. Archaediscidae, which have recrystallized wall structure, were common in shallow water, carbonates and calcareous shales. Fusulinacea, most of which probably had photosynthetic symbionts, were adapted to shallow carbonate depositional habitat at depths less than 15 to 20 m (49 to 66 ft). Carboniferous Fusulinidae and Ozawainellidae apparently occupied most of this depth range because of their adaptation to Middle Carboniferous cool surface waters. Permian Schubertellidae and Oazawainellidae became adapted to shallow, warm water lagoons and shelves. Verbeekinidae and Neoschwagerinidae were common in reef cores and upper flank deposits of Permian Tethyan reefs. Schwagerinidae also adapted to shallow to very shallow water carbonate environments, such as reef edges, shallow lagoons, tidal flat channels, margins of algal shoals and banks, and other shallow nearshore areas. For example, Eoparafusulina formed e tensive skeletal grainstones in many cross-bedded, subtidal deposits. Several globose lineages within the Fusulinacea possibly were pelagic, such as Robustoschwagerina, Pseudoschwagerina, Verbeekina, and many of the Permian Staffellidae. End_of_Article - Last_Page 543------------

Depositional-Tectonic Setting of Clastic-Hosted Lead-Zinc Sulfide Deposits: ABSTRACT

Clastic-hosted, stratiform lead-zinc sulfide deposits (MacArthur River H.Y.C., Mt. Isa, Broken Hill, Australia; Sullivan, Howard's Pass, Anvil Range, Jason-Tom, Canada) precipitated from exhaled, hydrothermal fluids in similar depositional and tectonic settings. Host rocks suggest sedimentation in anoxic water below storm waves in areas devoid of active bottom currents. Depths were at least 150 to 200 m as inferred from the absence of storm-influenced deposits. Such numbers are suggested by calculations involving fetch distances of the ancient basins and by End_Page 643------------------------------ effective wave base in modern seas. Supporting evidence comes from the lack of bottom-dwelling metazoans in Phanerozoic examples indicating anaerobic conditions. In the examples, laminated to thickly bedded sulfides are interstratified with shales, siliciclastic or carbonate-sediment gravity-flow deposits, and some cherts. Where not obscured by deformation and metamorphism, sandstones show grading, flutes and grooves, load casts, and Bouma sequences. Submarine mudflow units may be common. Lacking are hummocky cross-stratification, wave ripples, mudcracks, abundant medium-scale cross-stratification, or other evidence in ore zones of shallow-marine processes or of subaerial exposure. Thicknesses of clastics point to basin depths much greater than 200 m. These deposits apparently formed in extensional tectonic regimes. Stratigraphic thickness variations, facies changes (especially the presence of local, fault-derived slumps), or geophysical evidence suggest the presence in many of active faults during ore genesis. These faults formed the basins or formed local traps, provided conduits for hydrothermal fluids, and positioned convective cells. End_of_Article - Last_Page 644------------

Grand Rapids Formation, North-Central Alberta: An Example of Nearshore Sedimentation in a High Energy, Shallow, Inland Sea: ABSTRACT

End_Page 589------------------------------The Lower Cretaceous Grand Rapids Formation in the Wabasca Oil Sand Deposit of north-central Alberta contains approximately 10.5 billion cu m (66 × 109 bbl) of viscous, low gravity bitumen in the subsurface. It represents a progradational clastic sequence building northwest into the Boreal Sea during early Albian time. The formation measures up to 90 m thick and consists of three progradational sandstone units (C, B, and A members in ascending order), separated by two marine shale/siltstone units, indicative of marine transgression. Sequences of lithologies, sandstone geometry, and sedimentary structures indicate that each sandstone member represents a variety of depositional environments. Each shows a shallowing-upward trend from nearshore marine below effective wave base to lower and upper shoreface facies. Tidal inlet, beach, and possibly coastal dune environments are found in the upper parts of the A and B sandstones. Variations between localities indicate that upper shoreface to ?dune environments laterally grade into lagoonal and/or terrestrial environments. Coals, which locally cap the A and B sandstones, indicate back-barrier lagoon or interchannel marsh deposits. Local chert pebble beds found in the upper shale/siltstone sequence and in the basal part of the A sand are interpreted as possible major storm deposits. On a regional scale, the Grand Rapids Formation of the Wabasca area appears to represent a barrier island coastal complex possibly adjacent to deltaic or delta plain sequences. End_of_Article - Last_Page 590------------

Storm-Wave Genesis of Bioclastic Carbonates in Upper Jurassic Epicontinental Mudstones, East-central Wyoming

ABSTRACT The offshore mud facies of the Late Jurassic (Callovian-Oxfordian) North American epicontinental seaway is represented in east-central Wyoming by the Redwater Shale Member of the Sundance Formation. This member contains stratigraphically discontinuous concentrations of carbonate skeletal debris and fossiliferous mudstone within an otherwise siliciclastic lithologic association. Individual limestone beds thin and thicken laterally and range in thickness from a few centimeters to several tens of centimeters. Detailed petrographic analysis reveals several distinct carbonate depositional textures; grainstones (containing no mud-sized matrix grains other than remnant grain-sheltered patches), packstones and wackestones. These carbonate textures indicate multiple modes of formation. Fossil rains similar to those from carbonate lithologies occur, in much lower concentration, within surrounding siliciclastic mudstones. Muddy fabrics suggest that original skeletal accumulation took place under non-turbulent conditions. Subsequent winnowing during passage of greater-than-normal magnitude waves resulted in varying degrees of grain-support. Winnowing is attributed to turbulence generated near effective wave base, resulting in entrainment of grains and/or pressure gradients created by the passing wave form, which gently flush mud and interstitial waters from saturated bottom sediments. Wackestone and packstone fabrics are the result of incomplete winnowing of mud-sized matrix grains and form at greater distances from the effects of wave energy. Grainstone fabrics result from wave energies that either entrain bioclasts or cause a thorough flushing of mud from near-surface sediments. Progressively shoaling regressive conditions during upper Redwater Shale deposition facilitated the winnowing and concentrating effects of storm-wave interaction with the fossiliferous mud facies of topographically higher areas in the Late Jurassic sea. It is proposed that a variety of carbonate fabrics result from wave-caused winnowing effects, which vary with water depth and wave magnitude. Such fabrics are present in thin bioclastic carbonate units of the Redwater Shale Member of the Sundance Formation, and it is suggested that their origin is due to storm-wave winnowing of a slightly irregular muddy sea bottom. This mechanism and the depositional textures it produces are significant in the distinction between diastemic skeletal grain accumulations and wave-winnowed grainstones. The storm-wave process may also prove applicable in the re-evaluation of certain epicontinental marine deposits previously interpreted as classical turbidite lithologies. Given the vast expanse of geologic time, it is likely that catastrophic events, suc as intense storms and their related processes, have contributed significantly to the preserved sedimentary record.

Paleozoic Carbonate Facies of the Central Appalachian Shelf

ABSTRACT The central Appalachian Shelf, approximately bounded on the northeast by the Precambrian crystallines of the Adirondacks, the Cincinnati-Algonquin arch to the northwest, and including the western portion of a geosynclinal terrane lying to the southeast, received sediments discontinuously throughout the Paleozoic. Major carbonate sequences were deposited during late Cambrian-early Ordovician (Conococheague-Beekmantown), medial Ordovician (Black River, Trenton), medial Silurian (Lockport), late Silurian-early Devonian (Tonoloway, Keyser, Helderberg), and medial Devonian (Onondaga). Minor, but equally interesting, carbonate units were deposited in the medial Devonian (various thin limestones within the Hamilton), and early late Devonian (Tully). Recent environmental stratigraphic studies of these carbonate units show a great variety of lithofacies and biofacies (Conococheague, Wilson, 1952; Zenger, 1969; Beekmantown, Sando, 1957; Donaldson, 1969; Black River, Textoris, 1968; Walker, 1969; Trenton, Cameron, 1968; Lockport, Zenger, 1965; Tonoloway, Gwinn and Clack, 1964; Rondout-Keyser, Harper, 1969a, 1969b; Keyser-Helderberg, Head, 1969; Laporte, 1967, 1969; Onondaga, Lindholm, 1967; Portland Point, McCave, 1968; and Tully, Heckel, 1966). Despite this diversity, four major facies suites can be recognized. Tidal flat deposits consisting of laminated, dolomitic, mudcracked, intraclastic rocks with low faunal diversity and containing algal structures are well developed among the older carbonate units (Conococheague, Beekmantown, Black River, upper Lockport, Tonoloway, Rondout-Keyser, and lower Helderberg). These rocks represent supratidal and intertidal environments. Shallow subtidal deposits consisting of biomicrites (usually well burrowed), biosparites (frequently current stratified), and occasional oosparites, with relatively diverse and abundant biotas, are particularly common in the middle portion of the Cambrian to Devonian sequence (upper Black River, Trenton, middle Lockport, Rondout-Keyser, Helderberg, and Onondaga). These rocks record restricted to open marine environments lying above, or slightly below, effective wave base. Deep subtidal deposits consisting of well burrowed, argillaceous and quartz silty biomicrites with abundant and diverse biotas are more typical of the younger part of the sequence (Onondaga, Portland Point, and Tully). These rocks represent open marine environments below effective wave base. Carbonate build-ups are found throughout, from Cambrian to Devonian, and occur either as small algal mounds (Conococheague, Beekmantown, and Lockport), as tabulate or stromatoporoid biostromes (Black River, Lockport, Rondout-Keyser, Helderberg, and Tully), or as bioherms dominated by rugose and tabulate corals (middle Lockport, Helderberg, and lower Onondaga). Fossil diversity and abundance are usually high within the biostromes and bioherms. As might be expected, the temporal distribution of these broad facies suites parallels the tectonic history of the central Appalachians. Thus, during times of tectonic stability, carbonate tidal flat and shallow subtidal deposits were abundant. As tectonism increased in the bordering geosynclinal areas to the southeast the nearshore parts of these environments were flooded by land-derived clastics; greater subsidence of the shelf areas also seems to have occurred so that deeper-water carbonates became more common. Carbonate build-ups had somewhat more complex reasons for their development. While related to the general tectonic regime in that they are found near the more slowly subsiding, shallow parts of the basin, they were also dependent on good marine circulation and the local paleogeography.

Paleozoic Carbonate Facies of Central Appalachian Shelf: ABSTRACT

The central Appalachian shelf, which received sediments discontinuously throughout the Paleozoic, is bounded on the north by the Precambrian crystalline rocks of the Adirondacks, on the west by the Cincinnati arch and on the east by geosynclinal basins. Major carbonate sequences were deposited during the Late Cambrian-Early Ordovician (Conococheague, Beekmantown), Middle Ordovician (Black River, Trenton), Middle Silurian (Lockport), Late Silurian-Early Devonian (Tonoloway, Keyser, Helderberg), and Middle Devonian (Onondaga). Minor, but equally interesting, carbonate units were deposited during the Middle Devonian (several thin limestones within the Hamilton) and Late Devonian (Tully). Recent environmental stratigraphic studies of these carbonate rocks show a great variety of lithofacies and biofacies. Despite this great diversity, four major facies complexes can be characterized. Tidal flat deposits, consisting of laminated, dolomitic, mud-cracked, intraclastic rocks with low faunal diversity and algal structures, are well developed in the older carbonate units (Conococheague, Beekmantown, Black River, upper Lockport, Tonoloway, Keyser, and lower Helderberg). These rocks were formed in supratidal and intertidal environments. Shallow subtidal deposits, consisting of biomicrite (generally well burrowed), biosparite (in many places current stratified), and some oosparite with relatively diverse and abundant biotas, are particularly common in the middle part of the Cambrian-Devonian carbonate sequence (upper Black River, Trenton, middle Lockport, Keyser, Helderberg, and Onondaga). These rocks record restricted- to open-marine environments above, or slightly below, effective wave base. Deeper subtidal deposits, consisting of well-burrowed impure biomicrite with less abundant and less diverse biotas, are more typical of the younger part of this interval (Onondaga, Portland Point, and Tully). These strata formed in open-marine environments below effective wave base. Carbonate buildups are found throughout the Cambrian-Devonian either as small algal mounds (Conococheague, Beekmantown, Lockport), tabulate or stromatoporoid biostromes (Black River, Lockport, Keyser, Helderberg, Tully), or as bioherms dominated by rugose and tabulate corals (middle Lockport, Helderberg, and lower Onondaga). Fossil diversity and abundance are greatest within the biostromes and bioherms. As might be expected, the temporal distribution of these broad facies complexes parallels the Paleozoic tectonic history of the central Appalachians. Thus, during times of tectonic stability carbonate tidal-flat and shallow subtidal deposits were abundant. As tectonism increased in the eastern geosynclinal terranes, the nearshore areas of these environments were flooded by land-derived terrigenous clastics. Greater subsidence of the shelf areas also seems to have been general, with the result that deeper water carbonate became more common. Carbonate buildups seem to occur in a wider variety of environmental situations. Although related to the general tectonic regime, they also were dependent on good marine circulation and local paleogeography. End_of_Article - Last_Page 728------------

Petrology of the Jeffersonville Limestone (Middle Devonian) of Southeastern Indiana

Thin-section, polished-surface, and X-ray analyses of 500 samples collected from the Jeffersonville Limestone at 14 localities in southeastern Indiana are the basis for detailed correlation of carbonate facies and the reconstruction of sedimentary environments. Basal Jeffersonville sediments were deposited on Louisville Limestone (Niagaran) in southernmost Indiana. In south-central Indiana the Geneva Dolomite rose above the sea floor as a low-relief platform trending east-west. The existence of this platform was reflected throughout deposition of Jeffersonville sediments. The Jeffersonville is subdivided into five zones, each with characteristic fauna and carbonate rock types. The lowest, or coral zone, contains three distinct carbonate facies, including (ascending) grain-supported biomicrite, biosparite, and biomicrudite which overlap one another successively from south to north. The northern limit of sedimentation was controlled by the southern margin of the Geneva platform. The lower biomicrite and biosparite contain many upright branching corals, large corals, and both matlike and moundlike stromatoporoids. The upper biomicrudite contains a profusion of solitary corals and branching-coral fragments. These strata accumulated as a coral bank constructed below effective wave base. The Amphipora-zone sediments comprise biosparite, biosparrudite, biolithite, and biomicrudite in southern Indiana which grade northward into pelsparite which was deposited in shallow waters over the Geneva platform. These pelsparites grade northward into laminated dolomite. Regression of the Jeffersonville sea during deposition of upper Amphipora-zone sediments displaced the pelsparite and laminated dolomite facies southward. Fragments of Amphipora and matlike stromatoporoids are the dominant faunal elements. This zone reflects shoaling over the bank. The Brevispirifer gregarius zone contains grain-supported biomicrite in southernmost Indiana which grades northward into mud-supported biomicrite. The laminated beds continued to accumulate in the shallow shelf lagoon developed on the Geneva platform. Periodic exposure of the lagoon flats produced mud cracks. Charophytes grew in fresh- or brackish-water ponds on the flats, supplying oogonia to marine sediments to the south. Brevispirifer gregarius and echinoderms are the dominant faunal elements. Regression of the sea continued throughout deposition of this zone. The fenestrate bryozoan-brachiopod zone is composed of grain-supported biomicrite and biosparite which intertongue northward with the laminated beds of the shelf lagoon. Fenestrate bryozoans, echinoderms, and brachiopods characterize this zone. Corals and stromatoporoids reappear as significant faunal components. A second major transgression of the Jeffersonville sea is indicated by this zone. The Paraspirifer acuminatus zone is the most widespread of all the Jeffersonville zones and consists of grain-supported biomicrite and biosparite. These sediments are linked genetically to the same transgressive phase as the underlying fenestrate bryozoan-brachiopod zone sediments.