Offshore Facies Research Articles

Depositional Dynamics In A Mixed Carbonate–Siliciclastic System: Middle–Upper Cambrian Abrigo Formation, Southeastern Arizona, U.S.A.

Abstract The Middle and Late Cambrian mixed carbonate–siliciclastic Abrigo Formation of southeastern Arizona was deposited during the Sauk transgression in the craton interior landward of the passive margin of Laurentia. It overlies shallow-marine sandstone of the Bolsa Quartzite, which mantled the Precambrian land surface. The Abrigo Formation consists of ten distinct rock types that comprise fifteen lithofacies, which are grouped into eight facies associations. The strata represent an array of shallow-marine environments that were dominated by wave and storm activity. The interpreted paleoenvironments include lower offshore, upper offshore, offshore transition, and lower, middle, and upper shoreface. These environments migrated laterally as a function of relative sea-level changes along with input of siliciclastic sediment and its effect on carbonate deposition. Stratigraphic distribution of facies indicates that there were two separate carbonate factories: one in the nearshore immediately seaward of the siliciclastic shoreface and the other in the distal offshore area. Correlation across 170 km of the study area suggests that these lithofacies were deposited in six temporally distinct phases involving two transgressive–highstand systems tract couplets during the Bolaspidela through Crepicephalus biozones, a falling-stage systems tract in the Aphelaspis Biozone, and a lowstand systems tract in the Elvinia Biozone following the Sauk II–Sauk III hiatus. In general, the mixed carbonate–siliciclastic depositional environment of the Abrigo Formation shows that fine-grained siliciclastic facies dominate the transgressive systems tract. By contrast, carbonate sedimentation was dominant mostly during the middle to late phase of the highstand. The upper part of the highstand systems tract records progradation of the sandy shoreface. Nevertheless, the ratio between siliciclastic and carbonate sediment in various bathymetric zones differs from previously described inner-detrital-belt examples of Cambrian age. In the Abrigo Formation, some bioclastic grainstone in shoreface deposits contains siliciclastic sand, indicating that the two were deposited together until sand was dominant. However, carbonate production and siliciclastic mud sedimentation were for the most part mutually exclusive, suggesting that the shallow-water carbonate factory during the Middle and Late Cambrian was vulnerable to poisoning from clay or nutrient input. Consequently, carbonate sedimentation in the offshore transition, located between sand-dominated shoreface and mixed carbonate–siliciclastic offshore facies, reflects the bypassing of siliciclastic mud.

Types of soft-sediment deformation structures in a lacustrine Ploužnice member (Stephanian, Gzhelian, Pennsylvanian, Bohemian Massif), their timing, and possible trigger mechanism

The succession of Stephanian C lacustrine and fluvial facies of the Ploužnice member (Semily Formation) paleolake in the Krkonose Piedmont Basin (northern Czech Republic) preserved in borehole Sm-1 shows five types of soft-sediment deformation structures (SSDS): (1) sediment injections (clastic dikes), (2) load structures and ball-and-pillow structures, (3) water-escape structures (discontinuous laminations, deformed wavy bedding, recumbent folding, and dish or pillar structures), (4) deformations by growth of carbonate and silica minerals during diagenesis, and (5) bioturbation. Bioturbations disturbing mechanical SSDS suggest that soft-sediment deformations may occur syndepositionally or soon after deposition. The discussed mechanical SSDS are developed in a lacustrine environment, most being in lower shoreface and offshore facies. The mechanical SSDS found in the Ploužnice lake deposits occur in all lacustrine facies associations (290 SSDS horizons in a 67-m-thick succession). The cumulative thickness in cm of mechanical SSDS horizons per meter of thickness (ratio cm SSDS/m) is the highest in lower shoreface facies where it reaches from 50 up to 59.7 cm SSDS/m. Offshore facies association reaches 44 cm SSDS/m. Upper shoreface facies associations have 25.9 and 26.0 cm SSDS/m, while nearshore/mudflat facies associations preserve from 22.5 to 20.5 cm SSDS/m, and in palustrine carbonate, 13.5 cm mechanical SSDS/m was found. SSDS in fluvial facies are rare (2.9 cm SSDS/m). The distribution of SSDS in the Sm-1 borehole shows clear relationships to sedimentary facies and processes such as density flows or fluctuation of water level. The relationships of particular structures indicate a relative timing of formation which is as follows: sediment deposition was followed by the formation of mechanical SSDS, then by bioturbation, and finally by deformations due to early diagenetic growth of carbonates and silica. The distribution of SSDS in vertical sections and their direct relationship to sedimentary facies point to endogenic rather than external trigger mechanisms such as seismic activity. The main endogenic trigger mechanisms responsible for the origin of SSDS included a Stephanian semi-humid seasonal climate, basin morphology with relatively steep gradients, and elevated source areas, which created a conducive environment for rapid and repeated deposition of sheetfloods and hyperconcentrated flows and turbidites causing syndepositional loading.

Deep-water microbialites of the Mesoproterozoic Dismal Lakes Group: microbial growth, lithification, and implications for coniform stromatolites.

Offshore facies of the Mesoproterozoic Sulky Formation, Dismal Lakes Group, arctic Canada, preserve microbialites with unusual morphology. These microbialites grew in water depths greater than several tens of meters and correlate with high-relief conical stromatolites of the more proximal September Lake reef complex. The gross morphology of these microbial facies consists of ridge-like vertical supports draped by concave-upward, subhorizontal elements, resulting in tent-shaped cuspate microbialites with substantial primary void space. Morphological and petrographic analyses suggest a model wherein penecontemporaneous upward growth of ridge elements and development of subhorizontal draping elements initially resulted in a buoyantly supported, unlithified microbial form. Lithification began via precipitation within organic elements during microbialite growth. Mineralization either stabilized or facilitated collapse of initially neutrally buoyant microbialite forms. Microbial structures and breccias were then further stabilized by precipitation of marine herringbone cement. During late-stage diagenesis, remaining void space was occluded by ferroan dolomite cement. Cuspate microbialites are most similar to those found in offshore facies of Neoarchean carbonate platforms and to unlithified, buoyantly supported microbial mats in modern ice-covered Antarctic lakes. We suggest that such unusual microbialite morphologies are a product of the interaction between motile and non-motile communities under nutrient-limiting conditions, followed by early lithification, which served to preserve the resultant microbial form. The presence of marine herringbone cement, commonly associated with high dissolved inorganic carbon (DIC), low O2 conditions, also suggests growth in association with reducing environments at or near the seafloor or in conjunction with a geochemical interface. Predominance of coniform stromatolite forms in the Proterozoic--across a variety of depositional environments--may thus reflect a combination of heterogeneous nutrient distribution, potentially driven by variable redox conditions, and an elevated carbonate saturation state, which permits preservation of these unusual microbialite forms.

Mixed carbonate-clastic facies in the Eocene Kalatar Formation of the southwest Tarim Basin (NW China): tectonic and climatic controls

During the Eocene, the southwest Tarim Basin was a large epicontinental sea as a part of the Turun Sea in the eastern Paratethys. The marine succession of the Kalatar Formation is exposed along the piedmonts of the southern Tian Shan and western Kunlun Shan, and can be subdivided into three members. On the southern side of the Tarim Basin, a non-barrier shore-to-shelf, siliciclastic-dominated system, locally with carbonates and oyster biostromes, developed along the western Kunlun Shan, where river input led to a seaward arrangement of alluvial fan, foreshore, shoreface, and offshore facies. In contrast, on the northern side of the Tarim Basin, a carbonate ramp system formed along the southern Tian Shan, including shallow bioclastic-oolitic grainstone shoals, oyster beds, and red algal rudstones along with a number of evaporite units, which formed under hypersaline tidal-flat and lagoonal conditions. The basin center was a largely moderate deep and low-energy embayment facies. Deposition of the mixed carbonate-siliciclastic Kalatar Formation was controlled by tectonic activity, in particular the development of the Himalayan orogeny with the closure of Paratethys sea-level fluctuations, and arid-humid climate variations.

X-ray micro-CT reconstruction reveals eight antennomeres in a new fossil taxon that constitutes a sister clade to Dundoxenos and Triozocera (Strepsiptera: Corioxenidae)

Schaubcylindrichnus is an important trace fossil used in the characterization of offshore and shoreface depositional environments. Our understanding of this trace has followed a winding path including the introduction of several new ichnospecies and the subsequent recent synonymization into one ichnospecies. Likewise, our understanding of the ethological interpretation of the trace fossil has evolved from gregarious headdown deposit feeders to filter feeders, until it recently was shown that the tracemaker probably was a funnel feeder. Although several ichnospecies were recently synonymized into one ichnospecies, Schaubcylindrichnus coronus, variability in the number of tubes per sheaf and abundance of nested tubes, i.e., tubes that break up older tubes, were observed between different settings. These differences were studied in close detail at Miocene outcrops in Japan and Taiwan where the trace fossil occurs copiously and therefore allows the intraspecific morphological variability to be addressed. Around 2000 specimens from Japan and Taiwan were studied. The results show a distinct tendency for higher tube numbers in the offshore facies where sediments are characterized by higher silt/mud content, suggesting that S. coronus with higher tube numbers were constructed in a calm environment allowing longer dwelling periods. The increased abundance of nested tubes in settings characterized by thin sand layers indicate that the nested tubes are a reparation response to erosional events destroying the feeding funnels at the sediment-water interface. Consequently, tube number per sheaf and abundance of nested tubes may be used to further characterize the depositional environment in shoreface to offshore facies.

Diagenetically-enhanced trilobite obrution deposits in concretionary limestones: The paradox of “rhythmic events beds”

Certain offshore facies with extraordinarily well preserved fossils, particularly trilobites, display distinctive motifs of rhythmically bedded, decimeter-scale, concretionary limestone and dark gray shale. Examples are known from Cambrian to Neogene age, but are especially common in the lower-middle Paleozoic. The Hollardops trilobite beds of the Lower Devonian (Emsian) Khebchia Formation, Morocco, provide the exemplar of this phenomenon. Comparative examples include trilobite beds from the Upper Ordovician Katian of Oklahoma, New York State, and Ontario, Canada, and the Lower to Middle Devonian (Lochkovian–Givetian) of Tennessee, Oklahoma, and New York State. These Lagerstätten are observed to crosscut facies belts, occuring in both sparsely fossiliferous, dysoxic facies and more abundantly fossiliferous sections. Articulated fossils occur in both limestones and interbedded shales, indicating episodic deposition of fine grained sediments in both instances, but those in the shales are highly compressed and difficult to extract. Conversely, limestones yield particularly well preserved fossil material, including in situ lingulid brachiopods, and bivalves, cephalopods and complete outstretched, and enrolled trilobites. Articulated, uncompressed fossils may occur in varied orientations relative to bedding. This suggests that the organisms were locally entrained in mudflows, although occurrence of exoskeletal molt ensembles in some beds suggests that in certain events organisms could be preserved in situ. Enclosing sediments are typically strongly bioturbated indicating prolonged periods of sediment starvation following mudflow events but prior to early diagenetic cementation; however, most burrows are small and evidently did not disrupt entombed organism remains. Pyritic and calcite spar fillings in internal spaces of fossils formed early and rarely preserve remnants of lightly sclerotized tissues. Carbonate cementation occurred during interludes of low sedimentation; association with pyrite suggests formation within the zone of sulfate reduction.These intervals are particularly well developed in relatively sediment-starved, transgressive intervals and constitute a type of time-specific facies; these rhythmic obrution beds are especially characteristic of the Late Ordovician, and late Early Devonian (Emsian), where they occur on at least four paleocontinents. These beds formed selectively in warm, in semi-arid climates in which minor siliciclastics supplied from a relatively low relief hinterland were episodically re-suspended in slurries and transported offshore; they were favored in systems poised between deposition of siliciclastic and carbonate muds. The commonly rhythmic bedding of these deposits records a regular, recurring cyclic motif with overall durations of 10s of Kyr of carbonate redistribution superimposed upon muds that include abundant obrution deposits.

Rhodolith Stranding Event on a Pliocene Rocky Shore from Isla Cerralvo in the Lower Gulf of California (Mexico)

JOHNSON, M.E.; PEREZ, D.M., and BAARLI, B.G., 2012. Rhodolith stranding event on a Pliocene rocky shore from Isla Cerralvo in the lower Gulf of California (Mexico).Controls on present-day sedimentation around Isla Cerralvo in the southern Gulf of California provide a model for restricted Pliocene limestone distribution. The 10.46-km2 island is elongate and roughly parallel to the direction of prevailing north winds. Debris washed from deeply dissected valleys build fan deltas of sand-to-boulder size igneous clasts. These are transported south by long-shore currents, but the fans also create leeward zones with less agitated water. Remnants of a large Pliocene fan are exposed during low tide at Los Carillos on the SE side of the island. Adjacent is an unconformity between granite and granite-derived conglomerate with Nerita scabricosta. This extant gastropod is typical of the high intertidal rocky shore. The conglomerate is capped by a sandstone ramp with Argopecten abietus as an offshore facies. Basalt dikes exhumed from the granite formed natural groins that captured sediments and shells in the sand ramp. Whole rhodoliths, mostly 3.5 cm in diameter and 15-cm deep, covered 150 m2 within the ramp, now partly exposed among boulders in the basal conglomerate. Many rhodoliths encrust pea-sized rock cores. Accretion occurred in shallow water protected from extreme agitation by the nearby fan delta. Stranding of rhodoliths on the rocky shore was a storm-induced event.

Ontogeny and autecology of an Early Cretaceous trigoniide bivalve from Neuquén Basin, Argentina

Understanding ontogenetic variation is fundamental for adequate species definition and is the key for recognizing evolutionary relevant processes like heterochrony or developmental constraints. The larval and post-larval shell ontogeny and the ecology of the trigoniide bivalve Myophorella garatei are described in this paper. The species occurs in offshore transition facies of the late Valanginian Pilmatue Member of the Agrio Formation in the Neuquen Basin, Argentina. The larval shell has a rather small prodissoconch I; and if prodissoconch II is present, it is rather weakly developed. From these data, a larval stage of low dispersal capability is inferred for Myophorella garatei, either short-lived planktotrophic or nonplanktotrophic. The study of post-larval development included general shell shape and ornamentation. To assess general shell shape a geometric morphometric analysis was performed on the lateral view, together with a study of allometry for the shell width. As a result, two main allometric st...

A Major Eocene Sponge Fauna (Castle Hayne Formation, North Carolina)

Abstract The Castle Hayne Formation (Middle and Upper Eocene, North Carolina) contains a rich, diverse and well-preserved sponge fauna, equaled among known Tertiary faunas only by the Eocene fauna of Western Australia and the Miocene fauna of Algeria. It includes among Calcarea two genera of lithonines, (3 new species); among Demospongia one hadromerine, two genera of choristids (2 new species), one cephalorhaphiditid (new genus and species), and three genera and species of plinthosellid lithistids (2 new species); and among Hexactinellida four genera of hexactinosan hexactinellids (1 new genus, 1 new subgenus, 2 new species), two genera of lychniscosan hexactinellids (1 new genus, 2 new species), and a new genus and species of lyssacine hexactinellid. Noteworthy are whole fossil specimens of Geodia and Stelletta (Stolleya), two new genera (one of which is extremely abundant) of cup-shaped hexactinellids with radial fins, and the first fossil species of the Recent genus Verrucocoeloidea. Two faunal facies are recognized: a near-shore one dominated by demosponges and an off-shore one dominated by hexactinellids (with lithonine Calcarea), a pattern known from other published Cretaceous and Tertiary faunas. The off-shore facies is dated from associated fossils as late Claibornian (Auversian); the near-shore facies is probably of approximately the same age because of some common species. The prominence of the extinct plinthosellid lithistids and the absence of rhizomorines and megamorines is noteworthy. All the genera have close relatives in the Cretaceous; all but the plinthosellids have living relatives as well, and five genera are still extant. It is noteworthy that this Eocene fauna is richer than any known North American Cretaceous sponge fauna, even though the Cretaceous was elsewhere very sponge-rich.

Nearshore articulate crinoid from the Albian of Alberta, Canada (Early Cretaceous, Echinodermata)

During the middle Albian, a southward incursion of the Boreal Ocean flooded northern Alberta and adjacent British Columbia, forming a large embayment known as the Hulcross Sea. Marine mudstones of the Hulcross Formation and Harmon Member of the Peace River Formation record transgression, whereas sandstones of the Cadotte Member of the Peace River Formation record shoreline regression to the north. Abundant hummocky and swaley lamination in the Cadotte sandstone attest to the influence of storms on a shallow shelf. The Cadotte sandstone undergoes a lateral facies change from mud-free shoreface sandstone in the south to heterolithic offshore facies in the north. An articulated crinoid was found within a hummocky sandstone bed about 15 km seaward (north) of the shoreface-shelf facies transition. The articulated state of the crinoid indicates that it was buried very rapidly, and never exhumed. The arms through 20 mm of the column are preserved, but because the details of the aboral cup are not well preserved, this specimen must be left in open nomenclature. The elliptical columnals with a concave latus in the distal portion of the preserved column ally this specimen to the Bourgueticrinida, although with details of the aboral cup lacking and other characters atypical for Mesozoic bourgueticrinids, the Canadian specimen is placed in Bathycrinidae indeterminate. The oldest previously recorded bathycrinids were from the Upper Cretaceous (Maastrichtian), thus this report extends their range to the Lower Cretaceous (Albian).

The Holocene transgression as recorded by incised-valley infilling in a rocky coast context with low sediment supply (southern Brittany, western France)

Abstract A combination of morphobathymetric studies, very high-resolution seismics, core sampling and radiocarbon age data is used to investigate the latest stage of the sedimentary infilling of incised valleys in southern Brittany, related to the Holocene transgression. Owing to the bedrock morphology of this highly irregular rocky coast, two main types of valleys are defined by topographic rocky highs parallel to the coastline: 1) wide and rather shallow incised valleys offshore from a topographic sill, 2) narrow and relatively deep valleys between the sill and the coast (ria-type valley). The sedimentary infilling in both types of valleys becomes highly differentiated as the transgression advances onto the coastal area. In the wide valley seaward of the topographic sill, the infilling consists mainly of offshore heterolithic facies while, in the ria-type valley, most of the infill is composed of brackish mudflat deposits and estuarine tidal muddy sands. As the transgression proceeds, the rocky highs are flooded and the whole area is finally covered by the offshore facies. Radiocarbon dating indicates that: 1) the marine ravinement surface is highly diachronous (a few thousand years cross-shore); 2) the top of the offshore facies, coarser and very shelly, represents an episode of condensed sedimentation from about 3000 to 4000 years ago, amalgamating the maximum flooding surface (MFS) and the highstand systems tract (HST). However, we observe a muddy drape, strongly bioturbated in places, in the most proximal areas, overlying the offshore facies. It is thought to represent the modern and most recent stage of sedimentary infilling. This mud cover is made of fine-grained sediments of fluvial and biological origin, and is interpreted as a prograding HST. It reflects an increased influx, partly due to human activities. Finally, the main features of incised valley sedimentary infilling in a rocky coast context with low sediment supply can be characterized by (i) the very strong control of bedrock morphology, (ii) the diachronous character of the transgression, (iii) the late position of the MFS, and (iv), the highly reduced volume of the HST.

Le litofacies permiane e triassiche della formazione di M. Facito auctt. nell’area di M. Facito (successione di Lagonegro, Appennino meridionale)

Aim of this paper is to obtain a better definition of the stratigraphic succession of the M. Facito fm. that is the oldest part of the Lagonegro Mesozoic basin in Southern Apennines (Italy). Seismic data show that the Lagonegro succession overthrusted the western part of the Apulian Carbonate Platform and so the M. Facito fm. appears as a broken formation composed by a variety of different lithofacies. They were partially described by CIARAPICA & PASSERI (2000) in the two other typical areas. This paper, therefore, contribuetes to obtain a complete panorama of the lithofacies contained in the M. Facito fm. a) Calcirudites with fusulinids (Upper Permian). They are known only as scattered boulders and pebbles in tectonic melange and debris. These boulders are interpreted as tectonic shavings coming from melanges and reworked in the postorogenic debris flow. They are the only witness of the Permian in Southern Apennines. b) Capelluzzo lithofacies (Spathian-Aegean). It consists of black calcilutites, slates and calcarenites with abundant quartz grains. Sedimentary structures allowed us to recognize a coastal environment with offshore facies (thin-bedded black limestones), transition facies (limestones, slates and thin layers of cross-laminated quartz calcarenites) and shore-face facies (amalgamated beds of quartz calcarenites). c) Pietra Maura lithofacies (Anisian p.p.). It is represented by massive limestone with green algae, encrusting organisms, foraminifera and brachiopods. The main outcrops of Pietra Maura and Le Ripe are interpreted as carbonate build-ups. Sometimes they show evidence of exposure on the top. d) Varesano lithofacies (Anisian p.p.). It consists of marls, shales and sandstones. It partially lies by side and partially on the top of the previous lithofacies. The same species of brachiopods are contained in both the Pietra Maura limestone and in the shales and sandstones of Varesano, documenting their same age. Sandstones include quartz-wackes, quartz-psammites and quartz-arenites. In some outcrops they contain beautiful wavy ripples and burrows, but in other places they show turbidites with thick graded beds, flute casts, clay chips and current ripples. Sandstones with wave ripples overlying marls indicate a regressive trend. Turbidites are likely a witness of the successive deepening. e) Bellagamba lithofacies (Ladinian p.p.). This lithofacies marks the birth of the deep-water Lagonegro basin. It contains red nodular limestone (Rosso Ammonitico Auctt.) with very rare ammonites but rich in conodonts (early Fassanian to late Longobardian p.p.), radiolarian chert and true radiolarites. Calcirudites and megabreccias include elements coming from Ladinian build-ups that are not present in the Mt. Facito area but that are well known in other zones as Tempa di Rocca Rossa (Tempa di Rocca Rossa lithofacies, Ladinian p.p.). f) Petina Chiana lithofacies (late Longobardian to early Julian). This lithofacies is characterized by the presence of red shales and thin-bedded calcilutites with radiolarians and thin-shelled pelecypods (filaments). It contains the Ladinian-Carnian boundary and it marks the transition to the overlying Calcari con Selce fm. (Carnian and Norian).Some lithofacies lacking of clear relationships with the previous ones (Lithofacies Incertae Sedis) are also described in this paper. Among them, the Cognone lithofacies, that was settled down in a coastal environment and could be referred to the Anisian. The last subject is devoted to the description of the melange, well developed in the Mt. Facito area. It is possible to distinguish synsedimentary, synorogenic and postorogenic melange. The first one is represented by the Bellagamba lithofacies (radiolarites with calcirudites) that is the expression of the Middle Triassic rifting of the Ionian Ocean. The second one is related to thrust surfaces and it contains shavings of the oldest parts of the Lagonegro succession (Permian limestone and phyllites pebbles) sometimes mixed with Cretaceous elements of the «flysch galestrino fm.». The third one is very common and it consists of weathering products, loose sediments by mass flow and, sometimes, well cemented fluvial conglomerates. The abundance of melanges in the outcrops of Mt. Facito fm. probably lead to misinterpret it as a unique, huge olistostroma. In our opinion the fragmentary character of this formation is mainly due to tectonics

BURIAL HISTORY RECONSTRUCTION USING LATE DIAGENETIC PRODUCTS IN THE EARLY PERMIAN SILICICLASTICS OF THE FARAGHAN FORMATION, SOUTHERN ZAGROS, IRAN

In spite of the increasing importance to hydrocarbon exploration and production of the Palaeozoic succession in the Zagros area of SW Iran, few burial history and palaeothermal modelling studies of the interval have been carried out. This paper attempts to assess the burial and palaeotemperature history of the Lower Permian Faraghan Formation which is composed of stromatolitic dolomites overlain by mainly cross‐bedded sandstones. The formation grades up into the thick bedded carbonates of the Upper Permian Dalan Formation. The Faraghan and Dalan Formations are major hydrocarbon reservoir units in SW Iran and are time‐equivalents of the Unayzah and Khuff Formations in Saudi Arabia, respectively.The Faraghan Formation consists of shallow‐marine siliciclastics and foreshore deposits, including tidal‐flat and tidal‐channel, estuarine, sabkha, shoreface and offshore facies. In this study, diagenetic constituents are used to evaluate the formation's burial history in the Southern Zagros, an area for which only limited subsurface data is available. A burial history diagram for the formation was constructed for well Finu # 1 using WinBuryTM software. The diagram shows that the formation underwent progressive burial at variable rates between its deposition and the mid‐Tertiary, since when it has undergone rapid uplift.Burial diagenetic products in the Faraghan Formation comprise saddle, ferroan and zoned dolomites, together with dickite, illite/sericite and chlorite minerals. Additional burial‐related features include stylolites and dissolution seams. Isotopic signatures (δ18O versus δ13C) of the ferroan dolomites suggest a burial trend for the formation. Reconstruction of the paragenetic sequence together with the burial history diagram suggests a maximum burial depth of about 5000 m and a wide palaeotemperature range of 80‐160°C. However considering the saddle dolomites as a palaeothermometer, the temperature range narrows to 78 to 138 °C. The burial depth and temperature ranges closely correlate with the main stage of oil generation to the dry gas zone.

The relationships between Late Ordovician sea-level changes and faunal turnover in western Baltica: Geochemical evidence of oxic and dysoxic bottom-water conditions

The late Sandbian to the early Katian (Late Ordovician) in southeastern Norway is dominated by marine mudstones that contain an exceptional spectrum of macrofauna and trace element geochemistry, recording both abrupt and gradual faunal changes during major sea-level and environmental shifts. This study investigates the relationships between sea-level changes and their influence on the source of clastic material, oxygen levels in the bottom water and faunal changes, with special emphasis on the brachiopod fauna. Trace element ratios indicate that nearly stable upper dysoxic bottom-water conditions prevailed in the northwestern part of the Baltoscandian Sea. The exception is during the major shallowing in the earliest Katian, when there was an abrupt shift to oxic conditions as the sea bottom came within a normal storm wave base. Nonetheless, the pre-shallowing epibenthic fauna (though not the shelly endobenthic) in this area was rich and diverse and two major immigration phases of new brachiopod taxa are seen well before the shallowing. This indicates that the immigration of new taxa was not the result of an increase in oxygen content in the bottom water. More brachiopod genera stayed during the abrupt shallowing and increased oxic level than did during the following gradual return to deeper, dysoxic environments. The major brachiopod immigration phases took place markedly earlier in this northwestern part of the Baltoscandian Sea than in the central part (Eastern Baltic) and possibly also the comparable faunal turnover in Laurentia. The following disappearance of taxa during an early Katian transgressive event coincided with the faunal turnover in the shallow-water environments of the East Baltic. The depositional history was consistent within the investigated mudstone-dominated offshore facies of the Oslo Region, which are comparable to the Central Baltoscandian Confacies Belt of Baltoscandia. The composition of the siliciclastic material which was deposited in the basin was nearly constant through the sequence, though the land area expanded to include ophiolites, expressed by Cr enrichment, coinciding with the major sea-level drop. The Cr enrichment indicates that, by that time at least, the ophiolite complexes north of the Oslo Region became subaerially exposed. This enrichment thus forms a potential geochemical marker horizon representing the basal Katian Stage.

Palaeoecology of Jurassic encrinites: Reconstructing crinoid communities from the Western Interior Seaway of North America

Jurassic crinoid communities in North America are poorly known. However, encrinites (rocks chiefly composed of crinoid debris) are surprisingly common in North America, and represent some of the best examples of post-Palaeozoic crinoid accumulations. These rocks are highly significant for understanding the ecology of early isocrinid crinoids, and provide an insight into the wider crinoid communities of North America. Within this reanalysis of Jurassic crinoids of the Western Interior Seaway, a number of non-endemic forms are reported, together with some highly specialized crinoids. Encrinites are varied, with distinct marine and lagoonal communities recognized. These ecological associations are comparable to those seen in the European Jurassic, with many of the classic European genera being present. This new data, and existing data from the Jurassic shallow seas of Europe, demonstrates that crinoid distribution patterns are largely facies influenced while geographical factors are of limited importance. Even within the same region, taxa in lower energy offshore facies contrast strongly with those found in more restricted facies. However, very similar crinoid communities characterize comparable facies in North America and Europe.

Sequence stratigraphy and inferred relative sea-level change from the onshore British Jurassic

Sequence stratigraphy — the subdivision of successions into fades packages bounded by surfaces representing major shifts in depositional environment — has provided a systematic methodology for inference of relative sea-level change from vertical facies successions. In the present review, sequence stratigraphic work on exposures of British Jurassic successions is summarized. Resultant inferred relative sea-level curves for Early Jurassic and early Mid Jurassic successions show some strong similarities at the scale of ammonite zones between widely separated basins, implying sea-level change of at least regional extent or, alternatively, regionally co-ordinated changes in sediment supply. Also well developed are coeval stage-level influxes of sandy sediments into widely separated marine basins during the Late Pliensbachian, Late Toarcian—Aalenian and Oxfordian. Similarly, widespread synchronous shifts to more offshore facies are evident in the Early Pliensbachian, Early Toarcian, Early Callovian and Early Kimmeridgian.

Primary versus diagenetic bedding in the limestone-marl/shale alternations of the epeiric seas, an example from the Lower Lias (early Jurassic) of SW Britain

Micritic limestones form part of cyclic alternations of limestones- marls/shales in many offshore facies of epeiric seas and their origin has been, and still is, subject to controversy. As an example from the offshore ramp facies, the origin of Lower Lias micritic limestones of SW Britain has been investigated. The very continuous limestone beds show many petrographic and geochemical similarities to isolated nodules, which occur within the marls and are interpreted as being of diagenetic origin. Displacive growth of the microspar crystals was the most important process that occurred during the near-surface formation of the central part of limestone beds, but their edges formed during later burial. The carbon isotopic values (+1.1‰ to −3.4‰ (PDB)) of the central part of the limestone beds and nodules suggest a predominantly marine source for the carbonate, whereas bacterial sulphate reduction played a minor part in their cementation. The oxygen isotopic values (−1.5‰ to −7.2‰ (PDB)) of the characteristic limestone beds, and nodules suggest their formation (cementation/recrystallization) commenced at shallow depth but continued during progressive burial. The microspar crystals of the matrix of the limestones show rare aragonitic relics or pitted surface. Aragonitic fossil remains are rare in these offshore facies and this is in contrast to their abundance in the laterally equivalent shallow marine facies (Sutton Stone). It is suggested that early submarine dissolution of aragonitic components (lime mud and bioclasts) was important in providing carbonate for cementation of the limestones. Early submarine dissolution may have been a common process in epicontinental epeiric settings with low deposition rates, and as the result long residence time in the early diagenetic zones.

Sedimentary response to a tectonically induced sea‐level fall in a shallow back‐arc basin: the Mulichinco Formation (Lower Cretaceous), Neuquén Basin, Argentina

AbstractThis study examines the sedimentary response to a tectonically driven relative sea‐level fall that occurred in the Neuquén Basin, west‐central Argentina, during the late Early Valanginian (Early Cretaceous). At this time the basin lay behind the emergent Andean magmatic arc to the west. Following the relative sea‐level fall, sedimentation was limited to the central part of the Neuquén Basin, with the deposition of a predominantly clastic, continental to shallow marine wedge on top of basinal black shales. This lowstand wedge is called the Mulichinco Formation and consists of a third‐order sequence that lasted about 2 Myr and contains high frequency lowstand, transgressive, and highstand deposits. Significant variations in facies, depositional architecture, and internal organization of the sequence occur along depositional strike. These variations are attributed mainly to tectonic and topographic controls upon sediment flux, basin gradient, fault tilting, and shifting of the depocentre through time. These controls were ultimately related to asymmetrically distributed tectonic activity that was greater towards the magmatic arc in the west. The superposition of fluvial deposits directly upon offshore facies provides unequivocal evidence for a sequence boundary at the base of the Mulichinco Formation. However, the Mulichinco sequence boundary is marked by shallow, low erosional relief and widespread fluvial deposition. The surface lacks prominent valleys traditionally associated with sequence boundaries. This non‐erosive sequence boundary geometry is attributed to the ramp‐type geometry of the basin and/or rapid uplift that limited stratigraphic adjustment to base‐level fall. Significant along‐strike facies changes and a low‐relief sequence boundary are attributes that may be common in tectonically active, semi‐enclosed basins (e.g. shallow back‐arc basins, foreland basins).

Record of palaeoenvironmental changes in a Lower Permian organic-rich lacustrine succession: Integrated sedimentological and geochemical study of the Rudník member, Krkonoše Piedmont Basin, Czech Republic

This study presents an integrated sedimentological and geochemical analysis of the Lower Permian Rudník member — the most extensive lacustrine deposits in the Krkonoše Piedmont Basin. Grey to black and variegated lacustrine mudstones, laminites and carbonates of the Rudník member have a lateral extent of more than 400 km 2. In the studied sections four facies associations were recognised: A) anoxic offshore, B) suboxic to oxic offshore, C) nearshore and mudflat, and D) slope deposits. In the northern part of the E–W elongated basin, anoxic to suboxic organic-rich offshore lacustrine facies dominate and form a succession up to 130 m thick. Fan-delta and turbidite facies occur locally along the faulted northern basin margin. The central part of the basin is occupied by anoxic to oxic offshore facies interfingering with nearshore carbonate and mudflat facies of the low-gradient lacustrine margin. In the central part of the basin, the thickness of the lacustrine deposits of the Rudník member reaches up to 60–70 m. In the southern part of the basin fluvial and alluvial plain facies dominate and alternate with minor lacustrine nearshore facies. The lateral facies distribution indicates that subsidence along the northern basin fault was the main mechanism generating the asymmetric infill geometry in the basin's half-graben setting. The δ 1 8O values of primary and early diagenetic calcite range between − 11.0‰ and + 1.3‰ (V-PDB) and δ 1 3C values between − 5.1‰ and + 3.7‰; most of the data fall within the range of freshwater limestones. Coarser-grained pure microspar laminae show more positive δ 1 3C values in comparison to clayey organic-rich micrite laminae, and are interpreted as a record of bioinduced precipitation during seasonal eutrophication. The obtained δ 1 3C TOC values range from − 29.0‰ to − 24.0‰, the total organic carbon (TOC) content from 0.26% to 23%. Maceral analysis and Rock-Eval pyrolysis indicate that most of the samples contain a mixture of aquatic and terrestrial organic matter, but two minor, distinctive groups of samples with algally-dominated and terrestrially-dominated organic matter composition, respectively, were also found. The study of vertical changes in boron content in the clay fraction of the lacustrine mudstones shows that high lake level stages were periods of lower salinity with the lowest boron contents (from 73 to 268 ppm), and periods of falling lake level were followed by significant increases in salinity with much higher boron values (293–603 ppm). Lake-level fluctuations of the Rudník lacustrine system, which are recorded by shallowing-up units of sedimentary facies within most of the sections throughout the basin, can also be traced within the monotonous black shale dominated sections, where no sedimentological evidence of these lake level changes exists. Good indicators for such changes seem to be the δ 1 8O and δ 1 3C values of primary calcite, δ 1 3C TOC and HI. These lake level fluctuations are interpreted as driven by climatic oscillations in the order of tens of thousands years, which could reflect climatic changes connected with the last glaciation event of Gondwana.

Paleoenvironments and regional stratigraphic framework of the Middle-Upper Albian Lepine Formation in the Liard Basin, Northern Canada

Abstract The thick Middle to Late Albian Lepine Formation in the Liard Basin of northeastern British Columbia, southern Yukon and Northwest Territories preserves a continuous record of sedimentation during a time of multiple sea-level fluctuations that produced pronounced unconformities elsewhere in the Cretaceous Western Interior Seaway. Although sediment supply was high, the rapidly subsiding sub-basin produced sufficient accommodation to maintain offshore and shelf conditions throughout the interval. Using lithology, micropaleontology, well log correlations and Rock Eval pyrolysis, the 1000 m thick Lepine Formation is divided into five units in upper/lower offshore and shelf facies. The foraminiferal fauna differs substantially from that described from the Peace River Foothills and documents a continuous marine record during flooding of the Joli Fou Sea and the transition to the Mowry Sea. Regional well log correlations show significant thinning of the Lepine Formation southwards to the equivalent upper Buckinghorse and Hasler formations. Markers at transgressive surfaces can be traced for greater than 400 km along the foredeep. The thick, lower to middle part of the Lepine Formation (Units 1-3) contains foraminifera of the Ammobaculites wenonahae Zone and Ammomarginulina Assemblage Zone that record stressed oceanographic conditions related to periods of uplift west and northwest of the Liard Basin during the Joli Fou and lower Viking intervals. The transition between the two major inundations (Joli Fou and Mowry) is represented by a thick continuous interval in the Liard Basin. The Viking marker (lowermost Unit 3), traced in well logs and projected onto the outcrop section, has no obvious biostratigraphic definition. A major increase in foraminiferal diversity occurs near the base of the Bougie member (Unit 4), indicating more normal marine conditions within the Mowry Sea. A major flooding surface above the Bougie member is correlated with the 'Viking grits' (base of Unit 5) transgressive lag in the Peace River region of the Rocky Mountain Foothills.