Shallow Subtidal Environments Research Articles

The conodont fauna in the Middle Ordovician Eoplacognathus pseudoplanus Zone of Baltoscandia

Conodont faunas from the Eoplacognathus pseudoplanus Zone in five main areas of Middle Ordovician rocks in Sweden are described and discussed and compared with faunas of comparable age from neighbouring areas. The investigation is based on 118 samples which together yielded 175 114 conodont elements. The Swedish conodont faunas are intermediate between shallower-water faunas from the eastern part of Baltica and deeper-water faunas towards the west. Correlation with the Yangtze platform of south-central China is fairly easy, and the occurrence of key taxa, such as Histiodella, makes it also possible to correlate some levels with Laurentia. Although faunal lists for the Swedish localities are almost identical, the conodont taxa have their relative abundance maxima in separate areas: species of Protopanderodus and Periodon are most common in deeper-water settings, Eoplacognathus, Baltoniodus and Microzarkodina characterize intermediate areas, and Semiacontiodus is typical of shallow subtidal environments

Palynofacies variations in a carbonate ramp system environment (Upper Muschelkalk, NE France)

Abstract This study analyses variations in the size of black woody phytoclast and palynofacies types of shallow subtidal environments evolving to deeper subtidal environments, of latest Illyrdian-latest Fassanian age (Upper Muschelkalk). Some of the results display some discrepancies between elevated particle sizes that were supposedly deposited in a relative proximal source and the rich veryhachid palynofacies that are thought to represent distal marine environments. A case study of the succession in the Héming quarry, located in the Alsace-Lorraine trough (southwestern part of the intracratonic Germanic basin) reveals that : (1) the palynological assemblages are dominated by acritarchs belonging to micrhystridids, disaccate pollen and woody phytoclasts; (2) the abundance of phytoclasts, sporomorphs and marine groups is not correlated with lithologies; (3) the length of the processes of micrhystridids is not a useful parameter in defining palynofacies types. In carbonate ramp environments, many factors could have confused the palynological signals such as high-energy events and the water level on which the nutrient supply depended pro parte. All these factors seem to have been governed by climatic or tectono-eustatic events.

Sedimentology of liassic carbonates (Pirencik Tepe measured section) in the Aydincik (Içel) area, southern Turkey

Liassic carbonates are widespread in the Aydincik (Içel) area where they occur within the Tauride Orogenic Belt. They are represented by the Dibekli Member of the Jurassic to Early Cretaceous Cehennemdere Formation. Seven lithofacies have been delineated within the 489m thick Pirencik Tepe measured stratigraphic section.The facies are: (1) Dolomite (supratidal environment), (2) Microbial laminated limestone (intertidal environment), (3) Intraformational conglomerate (dominantly subtidal environment, (4) Megalodont-Orbitopsella-bearing micrite (shallow subtidal environment), (5) Gastropod-bearing micrite (restricted subtidal environment), (6) Mudstone/micrite (restricted shallow subtidal environment), and (7) Oolite-intraclast-bearing micrite/sparite (high energy intertidal environment).From base to top, the Pirencik Tepe measured stratigraphic section consists of predominantly dolomite, microbial laminated limestone, and cyclic carbonates which are composed of intraformational conglomerate, Megalodont-Orbitopsella-bearing micrite, and microbial laminated limestone. Among the cyclic carbonates, gastropod-bearing micrite and mudstone are irregularly present. The uppermost portion of the section is an oolite-intraclast-bearing micrite/sparite. The overall Liassic carbonate sequence shows a slightly deepening upward sequence in contrast to each cycle in the cyclicity. The average cycle duration is approximately 27–34kyr, which is due to the fifth order cyclicity. The cyclicity is mainly caused by sea level changes. In some areas, a tectonic effect was added to sea level changes, indicated by the lateral changes in the thickness of the intraformational conglomerate.

Análisis de facies de la Capa de Oolitos ferruginosos de Arroyofrío en la Sierra de Arcos (Jurásico, Cordillera Ibérica septentrional)

In this work we have studied the Arroyofrio Bed. This is a classic and largely studied ironstone unit in the Iberian Range. The age of this bed ranges from the Lower Callovian to Middle Oxfordian. This is a complex lithologic unit formed by irregular sedimentary events and several discontinuities associated with stratigraphic gaps of variable extent. In this work five facies containing ferruginous ooids are described and a preliminary study of the carbonate cements and Fe oolite facies is presented. This ironstone bed was formed near to an emergent high (Ejulve high). The oolites formed in a shallow marine subtidal environment. The facies and cement analysis shows that there were no long episodes of subaereal exposure at the Callovian-Oxfordian boundary. These data also demonstrate that these sediments were not deposited in deep marine conditions. The most probable hypothesis is that Fe ooids were formed in shallow subtidal and perhaps intertidal conditions and that they were eventually transported into more distal and relatively deep basinal areas.

Platform environments, microfacies and systems tracts of the upper Cenomanian-lower Santonian of Sinai, Egypt

Factors controlling grain composition and depositional environments of upper Cenomanian—Santonian limestones of Sinai are discussed. The mainly shallow-water, inner-platform setting investigated is subdivided into five major facies belts, each represented by several microfacies types (MFTs). Their lateral distribution patterns and their composition underline aclear relation between depositional environment and platform position. The facies belts include sandstones and quartzose packstones of siliciclastic shorefaces, mudstones and bioclastic wackestones of restricted lagoons, shallow-subtidal packstones with diverse benthic foraminifera and calcareous algae, bioclastic and/or oolitic grainstones of inner-platform shoals, and wackestones of deep open-marine environments. The microfacies distribution patterns of the Cenomanian-Santonian strata are evaluated with respect to local and regional large-scale environmental changes. While protected shallow-subtidal environments with only subordinate ooids and oncoids prevail during the late Cenomanian, high-energy oolithic shoals and carbonate sands occur locally during the middle and late Turonian. They were probably related to a change of the platform morphology and a reorganisation of the platform after a late Cenomanian drowning. In the Coniacian-Santonian, the lack of ooids, oncoids, and the decrease of calcareous algae versus an increase in siliciclastics indicate a shift to lower water temperature and to a more humid climate. Especially in the Turonian, the interplay between sea-level changes, accommodation, hydrodynamics, and siliciclastic input is reflected by lithofacies and biofacies interrelation-ships that are elaborated within individual systems tracts. In particular, increasing accommodation intensified circulation and wave-agitation and controlled the distribution of high-energy environments of the middle and upper Turonian trans-gressive systems tracts. During highstands protected innerplatform environments prevailed.

Marine carbonate facies in response to climate and nutrient level: The upper carboniferous and permian of central spitsbergen (Svalbard)

Carbonate-dominated successions of the Gipsdalen and Tempelfjorden Groups from Svalbard record a significant shift from Photozoan to Heterozoan particle associations in neritic settings during the late Palaeozoic. During the Bashkirian, benthic particle associations which included photoautotrophs such as phylloid algae (Chloroforam Association) characterised shallow subtidal environments. Most depositional settings which endured siliciclastic terrestrial input exhibited poorly diversified associations dominated by brachiopods, bryozoans and siliceous sponges (Bryonoderm Association). During the Moscovian to Asselian, highly diversified associations typified by various calcareous green algae,Palaeoaplysina, Tubiphytes, fusulinids, smaller and encrusting foraminifers (Chloroforam Association) prevailed in carbonate sediments from supratidal to shallow subtidal environments. During the Sakmarian and Early Artinskian, oolitic carbonate sands (Chloroforam Association) typified intertidal flats, whereas shallow subtidal environments were occupied by moderately diversified associations with fusulinids, smaller foraminifers, echinoderms and bryozoans (Bryonoderm-extended Association) and poorly diversified associations with echinoderms, brachiopods and bryozoans (Bryonoderm Association). During the Late Artinskian to Kazanian, poorly diversified associations characterised by brachiopods, echinoderms and bryozoans (Bryonoderm Association), and sponge-dominated associations (Hyalosponge Association) reigned within siliceous carbonates of intertidal and shallow subtidal environments.

Carbonate deposition in mixed siliciclastic–carbonate environments on top of an orogenic wedge (Late Cretaceous, Northern Calcareous Alps, Austria)

In the Middle Turonian to Lower Campanian (Lower Gosau Subgroup; LGS) of the Northern Calcareous Alps, Austria, in the highstand systems tract of mixed siliciclastic–carbonate depositional sequences, small carbonate shelves locally developed that were mainly controlled by a differentiated morphology of the substratum and siliciclastic input onto a storm-dominated shelf. The LGS was deposited above thrust nappes of the Alpine orogen. The transgressive systems tracts of depositional sequences consist of local fan delta or fluvial deposits, overlain by a succession mainly of siliciclastics that records deepening from paralic to outer shelf environments. Locally, rudist biostromes and coral–rudist mounds accumulated in lagoonal/bay to inner shelf environments, but were buried by paralic siliciclastics. Along coastal sectors of low siliciclastic input, because of a high-relief truncation surface at the base of the LGS, the transgressive systems tracts consist of an upward-deepening succession of clastic carbonates deposited from gravelly to rocky carbonate shores. The highstand systems tracts are dominated by siliciclastics, and record shoaling to inner shelf depths and, near the basin margins, to lagoonal to marsh environments. During highstand conditions, delta progradation combined with shoreline compartmentalization by headlands locally led to establishment of inner shelf compartments of low siliciclastic input. In these compartments, carbonate shelves developed. Along strike, the carbonate shelves were up to more than 10 km in length (beyond limits of larger outcrops), and about 1 km to, possibly, 10 km wide down dip. The carbonate shelves consisted of (a) an inner shelf belt with coral–rudist mounds, rudist biostromes and bioclastic sand bodies, (b) a dissipative shore zone of bioclastic sand bodies, (c) open lagoons/bays with radiolitid biostromes, and (d) narrow, micro-tidal flats or lithoclastic/bioclastic beaches. Up-section, inner shelf to shore zone carbonate parasequence tracts consist of a coral–rudist mound, a rudist biostrome, and of shore zone bioclastic limestones. Parasequence tracts deposited in lagoons/bays commonly shoaled incompletely, and mainly consist of more-or-less marly limestones deposited in shallow subtidal environments with radiolitid biostromes, substrata of bioclastic sand or lime mud, and with local mass accumulations of gastropods or of epibenthic non-rudist bivalves. The carbonate shelf successions are up to 100 m thick and, in vertical section, consist of stacked parasequence tracts that become thinner up-section and record a shoaling of mean depositional water depth. Carbonate deposition was confined to the actual inner shelf to tidal flat/beach compartment. Down dip, the coral–rudist mounds scattered along the seaward fringe of the carbonate shelves graded into small haloes composed mainly of disoriented, fragmented rudists and corals. Larger carbonate slope depositional systems were not individuated, and the carbonate shelves interfingered with and pinched out into inner shelf siliciclastics. The development of larger carbonate slopes was prevented by the low relief of the coral–rudist mounds, combined with high input and effective dispersal of siliciclastics ahead. Carbonate shelf progradation over a possible distance of up to a few kilometres was linked with aggradation of shelf siliciclastics.

Acritarchs and microfossils from the Mesoproterozoic Bangemall Group, northwestern Australia.

Three microfossil assemblages occur in the Mesoproterozoic Bangemall Group (1625-1000 Ma) of northwestern Australia, each occupying a different environmental and taphonomic setting. In peritidal environments, benthic prokaryotic filaments and spheroids of matting habit and small size were permineralized by early diagenetic silicification of stromatolitic carbonates. In shallow subtidal environments, benthic filaments of large size and nonmatting habit and planktonic sphaeromorph acritarchs with thin walls and moderate dimensions were compressed in mildly kerogenous shale. In deeper subtidal environments, planktonic megasphaeromorph acritarchs with thick walls were initially entombed in concretionary nodules in highly kerogenous shale and then permineralized by silica during later diagenesis. Taxonomic diversity and numerical abundance evidently decrease offshore. The three assemblages have typical Mesoproterozoic aspects: peritidal benthic habitats were dominated by Siphonophycus-Sphaerophycus-Eosynechococcus-Myxococcoides-Palaeopleurocapsa, shallow subtidal settings were occupied by Siphonophycus-Leiosphaeridia-Pterosphermopsimorpha-Satka, and offshore plankton consisted solely of very large chuarid acritarchs. Because of its taphonomic restriction to mid-intertidal stromatolites, the peritidal assemblage can be equated in microenvironment with a similar assemblage in the Neoproterozoic Draken Conglomerate, suggesting that ecological stasis at the community level can last for intervals up to 900 million years. In the deeper subtidal assemblage, the common chuarid has an unusual mode of preservation, in three dimensions in early diagenetic concretions, revealing that it possesses a thick multilamellate wall. Because of this distinctive ultrastructure, the new genus Crassicorium is erected for these fossils, which are among the oldest indubitable eukaryotes. Very large (34-55 micrometers in diameter) filaments from shallow subtidal habitats are assigned to the emended species Siphonophycus punctatum.

Taphonomic experiments on marginal marine foraminiferal assemblages: how much ecological information is preserved?

The most important taphonomic processes affecting the composition of benthic foraminiferal assemblages are transport and destruction of tests. The latter includes dissolution of calcareous tests. We have previously simulated nature experimentally by dissolving mainly shelf to deep sea samples in weak acid to give acid-treated assemblages (ATA). In this study we sampled the marginal marine environments of the Skagerrak/Kattegat and found that the main taphonomic process there is dissolution of calcareous tests. Consequently, the taphonomic processes occurring in this area represents a perfect natural analogue of what we suspect to be the main mechanism for the formation of some fossil agglutinated assemblages. The evidence for this is that some living assemblages dominated by calcareous forms become original dead assemblages (ODA) often consisting entirely of agglutinated species and equivalent to ATAs. The principal ATA is dominated by Miliammina fusca and this ATA can be derived from 10 different types of ODA including those that are strongly dominated by various calcareous taxa. This residual M. fusca assemblage represents a broad range of intertidal to shallow subtidal environments. On the other hand, marsh living assemblages are completely agglutinated and obviously give rise to agglutinated ODA/ATAs but not necessarily dominated by the same taxa. All the ATAs faithfully record the low species diversity characteristic of marginal marine environments. The principal conclusion is that in spite of the severe taphonomic loss, the preserved assemblages record much ecological detail. This is in accordance with similar investigations of deeper water assemblages and has important consequences for palaeoecology. This study demonstrates unequivocally that it is always essential to compare data on living and dead assemblages in order to determine the pathways to fossilisation. The use of total (living plus dead) assemblages in this area would give unrepresentative and misleading results.

Natural dissolution of modern shallow water benthic foraminifera: taphonomic effects on the palaeoecological record

Comparison of the living and dead assemblages of benthic foraminifera from the coastal areas of the Skagerrak–Kattegat reveals that, although there are considerable similarities in terms of the presence/absence of species, there are major differences in relative abundance at species level. Furthermore, the relative abundance of calcareous tests in the living assemblages is considerably higher than that of the dead assemblages (average values of 67 and 30%, respectively). Since the environments are microtidal, there is little transport of foraminiferal tests due to tidal currents so there are no introduced exotic species although there is local transport caused by waves and wave-induced currents. However, the major taphonomic process appears to be dissolution, of calcareous tests. This is manifested through etching and breakage of test walls, leading to complete decalcification which leaves a residue of organic linings of certain taxa (e.g., Ammonia beccarii), and a marked increase in abundance of agglutinated tests in the dead assemblages. Although dissolution is normal in marsh settings throughout the world, the Skagerrak–Kattegat area is unusual in the intensity of dissolution in the subtidal zone. Whereas most of the calcareous species occur throughout the investigated depth range (except marsh), two of the subtidal agglutinate species (Eggerelloides scaber and Ammotium cassis) show upper depth limits. Consequently, the agglutinated foraminifera are better indicators than calcareous foraminifera of upper depth limits in these shallow subtidal environments.

Response of larger foraminifera to mixed carbonate-siliciclastic environments: an example from the Oligocene-Miocene sequence of Kutch, India

Abstract The late Oligocene-early Miocene mixed carbonate-siliciclastic deposits of Kutch developed on the stable shelf of a peri-cratonic rift basin. It hosts a number of larger foraminiferal genera. The depositional environment of the sequence is interpreted and carbonate microfacies analysis has been carried out. The environmental and microfacies distributions of different genera and their response to energy conditions and terrigenous influx are examined. It is interpreted that the sequence was deposited in a wide range of environments, including back-reef lagoon, shallow subtidal seas, intertidal, swamps, tidal channels and supratidal. Larger foraminifera are recorded from the lagoon and shallow subtidal part of the sequence. Heterostegina and Spiroclypeus are found to occur in lagoon while Miogypsina, Lepidocyclina and Operculina occur in both lagoon and shallow subtidal environments. Archaias and Sorites prefer subtidal conditions. The distribution of different genera in relation to carbonate microfacies varies. Miogypsina, Archaias and Sorites occur in more microfacies than Heterostegina, Spiroclypeus, Operculina and Lepidocyclina. Larger foraminifera also differ in their susceptibility to clastic influx in the environment. Spiroclypeus and Sorites prefer a carbonate-rich environment while Miogypsina is most tolerant to terrigenous input. The genera Heterostegina and Spiroclypeus are found to be restricted to low energy environments but Miogypsina, Lepidocyclina, Operculina, Archaias and Sorites can tolerate agitated water conditions. It is interpreted that Heterostegina and Spiroclypeus are extreme K-strategists while Miogypsina tends toward r-mode opportunists. Between the two ends of the spectrum lie Lepidocyclina and Operculina. The life history of Archaias and Sorites is possibly comparable to Miogypsina.

Census Assemblages in Hard-Bottom Coastal Communities: A Case Study from the Plio-Pleistocene Mediterranean

Census assemblages from hard-bottom coastal environments have scarcely been reported in the fossil record. Late Pliocene-Pleistocene sediments from the marginal Mediterranean Almeria-Nijar Basin (SE Spain) contain fossils with exceptional preservation following catastrophic burial (obrution deposits). The assemblage, dominated by barnacles, bryozoans, coralline algae, mussels and other mollusks, colonized debris-flow boulders of a fan delta in a protected, shallow subtidal environment, probably a bay. The minimum time for paleocommunity development is estimated at 17 years based upon coralline growth-rates. Reactivation of fluvial processes in the area caused anastrophic burial and instantaneous death; the biota remained in situ and permanently isolated from post-burial reworking. Taphonomic bias due to organismic escape from burial and sediment mixing by bioturbation is negligible. Dissolution of skeletal aragonite and loss of non-skeletonized organisms seem to be the main post-burial biases altering the original community. The biotic composition of hard-bottom coastal communities has remained almost unchanged since the Late Oligocene, at least at high taxonomic level of skeletonized organisms, indicating coordinated stasis. This represents an ecological-evolutionary unit for the Neogene and Quaternary.

Palaeoenvironmental interpretation of a Triassic-Jurassic boundary section from Western Austria based on palaeoecological and geochemical data

A section spanning the Triassic-Jurassic boundary is described from near the village of Lorüns in the Vorarlberg region of western Austria. At Lorüns, the uppermost Triassic is characterised by bedded carbonates of the Kössen Formation supporting a stenotopic fauna indicative of a shallow sub-tidal environment of normal marine salinity. The Triassic-Jurassic boundary may be represented as a sequence boundary developed on top of a 1.1 m thick red mudstone of the lower Schattwald Shale, which is interpreted to have been deposited in a marginal marine environment, possibly a mud flat. Above the boundary beds, the upper Schattwald Shale is characterised by thin-bedded marl and dark limestone beds with an earliest Hettangian macrofauna dominated by epifaunal filter-feeding bivalves, including ostreids, mytilids and oxytomids, which suggest a shallow, subtidal, salinity-controlled environment typical of an interplatform lagoon. Carbonate production rejuvenated in the later Early Hettangian with development of the Lorüns oolite, a shallow subtidal oolitic and oncolitic unit bearing echinoderms indicative of normal marine conditions. Low ThU ratios from the remainder of the section are a result of reduced thorium in carbonate-rich sediments and not from authigenic uranium in anoxic sediments. In the boundary beds evidence for marine anoxia (or dysoxia) is absent where ThU values, determined by gamma-ray spectrometry, are above 5. The negative excursion in δ13C and positive excursion in δ18O in the boundary beds may be due to secondary geochemical effects, due to organic diagenesis or the precipitation of caliche during paleosol development. Alternatively, the excursions may reflect a primary geochemical signal recording short-term decline in primary productivity. Comparison in δ18O and δ13C values between the Kössen Formation and Lorfüs oolite indicate no significant long-term geochemical changes are evident in the section and suggest that any environmental perturbations were restricted to the boundary beds and possible sequence boundary.

Thalassinoides and the Enigma of Early Paleozoic Open-Framework Burrow Systems

The trace fossil Thalassinoides, a common constituent of the Zoophycos and Glossifungites ichnofacies, is abundant in post-Paleozoic rocks. Modern Thalassinoides-like burrows are constructed by a variety of marine organisms, most importantly decapod crustaceans such as thalassinid shrimp, in modern intertidal and shallow subtidal environments. Lower Paleozoic examples of Thalassinoides are poorly documented despite the abundance of Ordovician mottled shelf limestone that is generally thought to represent dense populations of Thalassinoides burrows. The paleontological record of decapods begins only in the Devonian, and thus, a considerable time gap exists between the early occurrences of Thalassinoides burrows and the oldest fossils of organisms most commonly associated with their construction, particularly in post-Paleozoic forms. Specimens of a new ichnospecies of Thalassinoides from the lower Paleozoic Manitou and Peerless formations of Colorado, herein defined as Thalassinoides horizontalis, consist of bedding-parallel polygonal networks of smoothwalled, unlined, horizontally branching burrows. Burrows form both Yand T-junctions but contain no swellings at junctions or elsewhere. They consist of a narrow diameter tube or inner core (<4 mm) and a 3-4 mm outer wall, the latter representing a diagenetic halo formed around a mucus-impregnated burrow. Five styles of preservation relate to the presence or absence of burrow-fill and patterns of mineral replacement and weathering. The extremely small burrow diameters, lack of some features such as scratchings and swellings, absence of vertical shafts, and the common regularity of branching separates this new ichnospecies from other Thalassinoides ichnospecies in either Paleozoic or post-Paleozoic strata. Thalassinoides horizontalis burrows were not made by large decapod crustaceans like those that made postPaleozoic Thalassinoides, but by small crustaceans or, more likely, soft-bodied organisms that made semi-permanent to permanent tunnels within firmground sediment. The open-framework burrow systems may record the activities of some of the oldest vagile suspension feeders to colonize infaunal habitats.

Possible new evidence for the origin of metazoans prior to 1 Ga: Sediment-filled tubes from the Mesoproterozoic Allamoore Formation, Trans-Pecos Texas

Possible new evidence for the origin of metazoans prior to 1 Ga comes from sediment-filled tubes preserved on a bedding-plane parting in a chert in the Allamoore Formation (Mesoproterozoic) in a talc quarry in the Millican Hills, near Van Horn, Trans-Pecos Texas. Available data for the Van Horn region indicate that the age of the Allamoore Formation is ∼1250 Ma. The sediment-filled tubes are most likely the feeding traces of animals grazing on algal mats in shallow subtidal or intertidal environments.

Sedimentology and Diagenesis of the St. Peter Sandstone, Central Michigan Basin, United States (1)

The Middle Ordovician St. Peter Sandstone occurs at depths between 1600 and 3600 m in the central Michigan basin. Integration of conventional core and wireline log studies indicates that the formation consists of, from base to top, (1) up to 250 m of sandstone deposited in intertidal and supratidal sand flats with associated dolomitic lagoonal deposits, and shallow subtidal shoreface environments, (2) sandstones deposited in subtidal shoreface to upper offshore environments, and, (3) dolomitic and argillaceous sands deposited in a storm-dominated epeiric sea. Sandstone composition is closely related to depositional environment: quartz arenites occur in higher energy littoral facies, whereas feldspathic and carbonate-rich sands occur in predominantly lower energy shelfal f cies. Important modification of primary mineralogy and porosity occurred during diagenesis. A generalized, basinwide model for paragenesis includes (1) early calcite marine cement, (2) syndepositional dolomitization, (3) widespread precipitation of quartz and K-feldspar overgrowths, (4) pervasive replacement of early carbonate by burial dolomite, (5) local dissolution of unstable framework grains (e.g., K-feldspar) and carbonate cement, (6) growth of authigenic clay, and (7) pressure solution and additional precipitation of quartz overgrowths. Authigenic clays apparently formed along with or subsequent to economically significant secondary porosity. The different pathways of sandstone diagenesis observed in the St. Peter Sandstone are largely dependent on primary textures and composition inherited from the environment of deposition rather than other factors, such as depth of burial or position in the basin. The relationships among primary textures and composition, diagenesis, and reservoir sandstone properties is useful for prediction of reservoir quality in the St. Peter Sandstone in the Michigan subsurface. Weakly cemented sandstone reservoirs characteristic of high-energy shoreface and shallow offshore depositional environments have the best reservoir quality in the basin. Clay-cemented sandstone reservoirs deposited on offshore shelves have the poorest reservoir quality. Incompletely quartz-cemented sandstone reservoirs, comm n in intertidal/supratidal facies, have intermediate reservoir quality.

Silurian trace fossils in carbonate turbidites from the Alexander arc of southeastern Alaska

Early to Late Silurian (Llandovery‐Ludlow) body and trace fossils from the Heceta Formation of southeastern Alaska are preserved in the oldest widespread carbonates in the Alexander terrane. These fossils represent the earliest benthos to inhabit diverse shallow and deep subtidal environments in the region and are important indicators of early stages in benthic community development within the evolving Alexander arc. The ichnofossils are particularly significant because they add to a small but growing body of knowledge about trace fossils in deep‐water carbonates of Paleozoic age. Carbonate turbidites that originated along a deep marine slope within the arc yield a low‐diversity suite of trace fossils consisting of five distinct biogenic forms. Simple burrows (Planolites, two forms), ramifying tunnels (Chondrites), and tiny cylindrical burrows (?Chondrites) represent the feeding activities (fodinichnia) of pre‐turbidite animals that burrowed in the lime mud before the influx of coarser sediment deposited ...

Patellogastropods (Mollusca) from the Eocene Tejon Formation of southern California

Collections from the basal part of the marine Eocene Tejon Formation, Tehachapi Mountains, southern California, reveal new stratigraphic occurrences of patellogastropod limpets. Reports of these gastropods from California Paleogene strata are uncommon. The Tejon limpets are all members of the tropical genus Patelloida. Four taxa are represented in 33 specimens from five localities. These taxa include: mature specimens of P. tejonensis (Gabb, 1869), the first occurrence of the Eocene P. vokesi (Hickman, 1980) outside of Oregon, and P. triquetrus n. sp., described herein. Our study shows that a hypotype referred to P. tejonensis by Anderson and Hanna in 1925 is actually a fissurelloidean species, and we tentatively reallocate it to the genus Megathura Pilsbry, 1890. Analysis of depositional settings indicates that nearshore, shallow-subtidal depositional environments have higher patellogastropod abundance and diversity than intertidal or offshore-subtidal depositional environments. Morphological convergence between Tejon and Recent patellogastropod taxa is documented. Resemblance of the Eocene P. triquetrus n. sp. and an indeterminate Patelloidea sp. to the Quaternary “Collisella” scabra (Gould, 1846) and Lottia digitalis (Rathke, 1833), respectively, is remarkable.

Carbonate lithofacies and depositional environments of the Lower Eocene Farafra Limestone, Farafra Oasis, Western desert, Egypt

The Lower Eocene Farafra Limestone is subdivided into seven carbonate lithofacies. These lithofacies were deposited in three environments: off shore bank, bank interior and shelf slope. The lithofacies and the depositional environments in which they were deposited are: Lithofacies Environment Nummulitic-echinoidal wackestone Dolostone Ostrcod-miliolid wackestone Alveolina-miliolid packstone Bank interior Algal-alveolina-packstone Bioclastic packstone Bank interior Argillaceous foraminifera wackestone The off shore bank lithosfacies were deposited over and around structural highs (anticlines). Dolostones occupy the crets of the structural highs where they were formed by the dolomitization at mean sea level and/or in tidal zone. The Nummulitic-echnioidal wackestones were deposited around the structural highs in a warm shallow subtidal environments. The interior represents restricted marine waters it was enclosed between two off shore banks and was deposited in the structural low (syncline), hence the quiet water conditions and shallow depths. This interpretation stems from the facies associations which are mostly of ostracod-miliolid wackestone, alveolina-miliolid packstone and algal-alveolina packstone. The shelf slope lithofacies were deposited in a deeper setting away from the off shore banks at both extremes of the Farafra Oasis. The dominant lithofacies are bioclastic packstone and argillaceous foramineferal wackestone. The bioclasts are mostly of reworked fossils which were derived from the shallow off shore carbonate banks.

Depositional environments and diagenesis of upper jurassic subsurface sponge- andTubiphytes reef limestones: Altensteig 1 well, Western Molasse Basin, Southern Germany

An up to 300 m thick Kimmeridgian and Tithonian “reef sequence”, occurring in the Altensteig-1 well southwest of Augsburg, Bavaria, has been studied with regard to facies types, depositional environment and diagenesis. The reef sequence consists of oolitic sands and sand bars, stabilized by binding organisms and overgrown by siliceous sponges and the enigmatic organismTubiphytes. The biota consists of siliceous sponges, sclerosponges, bryozoans, brachiopods, molluscs, serpulid and terebellid worms, and echinoderms. Foraminifera exhibit no significant distribution patterns. Algae occur in samples of core 4 (Malm zeta 3). Four facies types (sponge/algal boundstones, tuberoid-peloid wackestone/packstone, ooid-intraclast grainstone andTubiphytes packstone/boundstone) can be differentiated. TheTubiphytes boundstones represent a particular reef type not yet described in detail from the Upper Jurassic. As can be deduced from subsurface samples and from surface samples (Graisbach quarry near Donauworth),Tubiphytes formed an organic frame-work within a shallow subtidal environment. The diagenesis of most of the section is characterized by the existence of early marine-phreatic cements, except for the Malm delta/Malm epsilon interval. For this interval, an widespread subaerial exposure of Upper Jurassic carbonates is indicated by the absence of marine-phreatic cements, the occurrence of meteoric-vadose meniscus cements, meteoric-phreatic granular to blocky cements, and of asymmetric radiaxial-fibrous cements.