Shallow Subtidal Settings Research Articles

Cambrian-Ordovician trace fossils of the Basissletta region, northeast Spitsbergen, Svalbard

Trace fossils of the Cambrian–Ordovician sedimentary succession of the Basissletta region, northeast Spitsbergen, Svalbard, belong to three ichnoassemblages. The first one, composed of Monocraterion and Diplocraterion, belongs to the Skolithos ichnofacies and characterises sandstones of the Tokammane Formation (lower Cambrian: Terreneuvian) deposited in a shallow subtidal setting. The overlying carbonates with pseudomorphs after evaporites (uppermost part of Terreneuvian and the Cambrian Series 2) display abundant Balanoglossites. These beds were deposited in an oxygenated but hypersaline environment. The younger Ordovician carbonates of the Kirtonryggen Formation and the mixed carbonate-clastic deposits of the Valhallfonna Formation display a more diverse but poorly preserved trace fossil suite (Phycodes, Curvolithus, Planolites, Palaeophycus) typical of the archetypal Cruziana ichnofacies. Some parts of the succession are not bioturbated, and these are characterised by dark coloured fine-grained sediments with primary lamination. They were mostly deposited between the normal and storm wave base or below the storm wave base in oxic to anoxic environments.

Faunal and paleoenvironmental changes at a Cambrian (Jiangshanian; Steptoean–Sunwaptan boundary interval) trilobite extinction event, in contrasting deep- and shallow-subtidal settings, Nevada and Oklahoma

Abstract Successions in Oklahoma and Nevada record trilobite extinction and replacement near the Steptoean–Sunwaptan boundary in inner-shelf and outer-shelf settings, respectively. Prior to the extinctions, different trilobite biofacies occupied these environments, but faunas became similar in composition across the environmental gradient in the overlying I. “major” and Taenicephalus zones. Faunal changes in the outer shelf at the I. “major” Zone begin at a drowning unconformity that brought dark, laminated calcisiltite and silty lime mudstone above a subtidal carbonate succession. In contrast, Oklahoma shows facies continuity in a succession of tidally influenced bioclastic carbonates. Loss of genera and a dramatic abundance “spike” of Irvingella are features of the I. “major” Zone in both regions. Turnover of biofacies occurred in the succeeding Taenicephalus Zone, with both the inner and outer shelf dominated by Orygmaspis (Parabolinoides). Blooms of orthid brachiopods in shallow water settings are underappreciated signals of faunal change in the extinction interval. Although absent from the outer shelf in Nevada, orthids became abundant enough in Oklahoma to form shell beds in the lower Taenicephalus Zone, but became rare in overlying strata. Carbon isotope stratigraphy includes a modest positive δ13C excursion that peaks in the extinction interval at 1.4‰ (Oklahoma) and 2.2‰ (Nevada), which is congruent with previous reports from Utah and Wyoming. Although consistent with regional upwelling of dysoxic waters, the absence of sedimentary evidence for significant environmental change over much of the shelf is problematic. This suggests that physical environmental change acted primarily as a catalyst for cascading ecological and biogeographic effects.

Microboring variability in bioclasts from a carbonate platform and a mixed carbonate ramp: Paleoenvironmental implications (Upper Triassic, Slovenia)

Decompositors of both organic matter and biogenic carbonates represent important, yet overlooked organisms that significantly contribute to biogeochemical cycles within carbonate ramps and platforms. Moreover, the distribution of the microboring communities in carbonate depositional settings exhibits characteristic zonal patterns controlled by various paleoenvironmental factors, such as water dynamics, sedimentation rate and consequently the time available for colonization. The aim of this study was to test this assumption in for the shallow-water, Upper Triassic (Carnian–Rhaetian) carbonates of the Southern Alps in Slovenia representing two different depositional carbonate environments. Both were deposited close to the northwestern margins of Neotethys in: 1) peritidal to shallow subtidal setting of mixed carbonate ramp and 2) intertidal part of a carbonate platform, or an open lagoon in a close proximity to a reef complex, both periodically affected by storms. Microboring communities were studied from thin section combining microscopical observations with image analysis and statistical approach using shape descriptors. This represents a novel approach to the ichnofacies studies as a supplementary technique to the well-established sedimentological and microfacies analyses. Microborings found in proximal areas of the restricted shelf are relatively small and highly circular. Even smaller microborings are typical for the more distal facies. Additionally, they occur here far less frequently than in the proximal facies. Larger and more common microborings are found in the intertidal zone, which probably resulted from the prolonged exposure on the sea-floor close to the carbonate platform.

Biological mats in siliciclastic sediments of the Paleoproterozoic Gunflint Formation, northwestern Ontario, Canada

The Gunflint Formation of northwestern Ontario, Canada, contains an extensive array of stromatolite morphologies and associated fossilized bacteria. It, and correlative units in the United States, provided some of the most persuasive early interpretations of stromatolites and evidence of Precambrian bacterial life. This study examined the siliciclastic rocks in the Gunflint Formation and discovered a multitude of features formed by the development of cohesive biogenic mats on bedding surfaces. In former shallow subtidal depositional settings, evidence of mat erosion was most common, with the presence of various types of wrinkle structures. Microscopically carbonaceous layers and rip-up fragments representing mats and their eroded remnants are well preserved. This emphasizes the abundance of bacterial life in the shallow nearshore of the Gunflint Formation about 1.88 billion years ago and further indicates an increased flux of reductants was necessary during this time period to establish low oxygen levels in the ocean.

An epeiric glass ramp: Permian low-latitude neritic siliceous sponge colonization and its novel preservation (Phosphoria Rock Complex)

Glass ramps are shallow-marine depositional settings in which siliceous sponge meadows dominate coastal environments. They are increasingly recognized throughout the Phanerozoic and represent a biosiliceous counterpart to neritic carbonate factories. Detailed reexamination of the Permian Tosi Chert in the Bighorn Basin indicates that it records a glass ramp that extended over at least 75,000 km2. Outcrops, cores, and wireline logs are used to discriminate previously unidentified shallow subtidal to peritidal facies in its landward extent. These facies indicate that sponge meadows ranged from variably oxygenated offshore settings through low-energy, well-oxygenated, and saline shallow subtidal settings, with spicules transported into supratidal environments affected by enterolithic evaporite growth. This range of subenvironments is largely unique among glass ramps. This is the result of the Tosi's accumulation in an epicontinental sea where waves impinged offshore but frictional attenuation caused low-energy nearshore environments. As a result, the Tosi shares similarity with epeiric sea carbonate deposition and is referred to herein as an epeiric glass ramp. The low palaeolatitude of the Tosi and hot and arid desert it bordered also contributed to its uniqueness as shallow waters were warmer and more saline than higher-latitude counterparts. As a result, a minor sea-level fall at the termination of biosiliceous deposition was associated with increased lagoonal circulation and refluxing brines that caused evaporite and dolomite precipitation within the upper Tosi. Preservational attributes of the Tosi also add to the range of unique traits that can be used to reconstruct neritic biosiliceous environments. These include three disparate colours of chert (black, grey, and purple) related to the host strata and diagenetic redox conditions, early chertification that preserved sedimentary structures within nodules, and nodule shape related to bioturbation intensity. The Tosi glass ramp thus expands the known extent and context of Permian glass-ramp deposition along the western Laurentian margin and illustrates key properties that will aid future glass ramp identification.

Semi-isolated, flat-topped carbonate platform (Oligo-Miocene, Sardinia, Italy): Sedimentary architecture and processes

The Chattian-Aquitanian carbonate system of Isili sub-basin (SE Sardinia, Italy) were studied to better understand the hydrodynamic processes controlling the formation of landward-downlapping, rhodolith-rich, giant clinoforms.The studied flat-topped platform was attached to an island (semi-isolated) and migrated onshore (landward) over a shallow marine, protected embayment. The depositional profile is characterized by four, sea to land, zones: seaward slope, flat-topped platform (submerged flat), landward slope and mixed siliciclastic-carbonate embayment. In particular, these zones record different levels, from high to moderate, of hydrodynamic energy and persistency. The paleodepth of the studied succession, based on T/D test on Amphistegina specimens, red-algal genera and geometric reconstruction of the overall system, ranges from −10 to −30m for the platform up to around −40m for the landward slope base. Thus, the flat-topped platform was permanently submerged and an extensive carbonate (sea grass, algal and bryozoan-algal) factory along with oyster framestones developed. The widespread presence in the barren zones of rhodolith-to-shell rich small to very small compound dunes (high-energy zone) suggests that the flat-topped platform was periodically swept by unidirectional, landward-directed currents. These currents allow the formation of large-scale, rhodolith-rich clinobeds along the landward slope (washover fan-like system). The presence of rodolith pavements developed along the flat-topped platform at or slightly below the fairweather wave base suggests that clinobeds were formed in a nearshore, shallow subtidal setting. Finally, the shallow marine, protected embayment (moderate-energy zone) represents the widening distal reaches of the currents flowing or along the clinoforms or from the coast basinward (river floods and/or ebb tidal currents).Despite few tidal-related sedimentary structures such as bi-directional ripples and small compound dunes separated by finer-grained “drapes” or reactivation surfaces were observed, the prevailing processes acting over the platform are unidirectional, landward-directed currents possibly associated with longshore currents and/or wave actions. However, the resultant migration of the whole system onshore (landward) cannot be easily explained with storm or wind-related processes. Thus the studied flat-topped platform seems to be controlled by long term tidal regulation within a meso to macro tidal regime. In particular, such meso/macro tidal environments experience multiannual to multidecennial phases of stronger/weaker tidal range fluctuations resembling periods of relatively sea highs and lows with respect to the mean sea level (0m). All the (wind, storm, wave and tidal) currents sweeping the flat-topped platform were maxima during phases of strong tidal fluctuations generating erosion and sediment transportation over the flat and accumulation on the landward slope (clinobeds). Conversely, during phases of weaker tidal range fluctuations overall currents were minima, clinoforms did not develop and factories widespread re-colonized the submerged flat. Therefore, the studied platform developed in a current-dominated and tidal modulated setting.Finally, the studied carbonates of Sardinia suggest that the Sardinian seaway and the incipient Provençal basin during the Chattian-Aquitanian were, locally, capable to generate meso to macro tidal conditions.

How far did feedback between biodiversity and early diagenesis affect the nature of Early Palaeozoic sea floors?

AbstractLatest Precambrian to Early Palaeozoic biosphere evolution triggered changes in early diagenesis and carbonate precipitation which fed back to biodiversity through colonization of hard substrates. Progressive increase in the depth and intensity of bioturbation and bio‐irrigation lowered the zone of early carbonate cementation in the uppermost sediment column. This firstly led to a decline in the abundance of the flat‐pebble conglomerates which had been a common feature of Cambrian and Early Ordovician successions, replaced by the peak and subsequent decline in the Palaeozoic abundance of submarine hardgrounds. The availability of very widespread lithified sea floors in shallow subtidal settings during the Ordovician promoted a rapid expansion in sclerobiont diversity and contributed to the Great Ordovician Biodiversification Event.

Continental-scale biogeographic variation: provinces versus gradients in the Upper Ordovician of Laurentia

AbstractAlthough provinces are widely used to delimit large-scale variations in biotic composition, it is unknown to what extent such variations simply reflect large-scale gradients, much as has been shown at smaller scales for communities. We examine here whether four previously described Middle and Late Ordovician provinces on Laurentia are best described as distinct provinces or as biotic gradients through a combination of the Paleobiology Database and new field data. Both data sets indicate considerable overlap in faunal composition, with spatial patterns in Jaccard similarity, quantified Jaccard similarity, and nonmetric multidimensional scaling ordination structure that correspond to variations in substrate type, specifically from carbonate-dominated strata in western Laurentia to mixed carbonate–siliciclastic strata in the midcontinent to siliciclastic-dominated rocks in easternmost Laurentia. Because sampling was limited to shallow-subtidal settings, this gradient cannot be attributed to variations in water depth. Likewise, geographic distance accounts for only a quarter of the variation in faunal composition. This cross-continent faunal gradient increases in strength into the early Late Ordovician, and appears to represent increased siliciclastic influx into eastern Laurentia during the Taconic orogeny. These results raise the question of whether biogeographic provinces may be in general better interpreted and analyzed as biotic gradients rather than as discrete entities.

STARS OF THE AFTERMATH: ASTERIACITES BEDS FROM THE LOWER TRIASSIC OF THE CARNIC ALPS (WERFEN FORMATION, SAURIS DI SOPRA), ITALY

AbstractTrace fossils are important evidence of benthic activity, but they have received less study than body fossils for investigating the aftermath of the end-Permian extinction. There is therefore a need to document Lower Triassic ichnofaunas to understand their significance with respect to the end-Permian crisis. In light of this need, this paper describes a novel Lower Triassic ichnosite at Mount Pallone (Carnic Alps, Italy), where the Campil Member (Smithian) of the Werfen Formation (Griesbachian–Spathian) presents an abundant ichnofauna characterized by excellent preservation and low diversity. Documented ichnogenera include Asteriacites lumbricalis, Gyrochorte comosa, Diplocraterion habichi and Planolites beverleyensis. The ichnofaunal composition and the bioturbation style suggest a marginal marine paleoenvironment ranging from intertidal to shallow subtidal settings. Storm influence, hydrodynamic energy, sedimentation rate, freshwater input and/or water temperature played an important role in structuring the benthic ecosystem. Dense (300 specimens/m2) aggregations of the trace fossil Asteriacites lumbricalis reveals social behavior of their inferred brittlestar producers (Echinodermata, Ophiuroidea). In line with modern brittlestar beds, social behavior provided significant advantage because raised arms of brittlestars dampened hydrodynamic energy. This study suggests that Asteriacites beds may be considered ichnological proxies for marine settings, low bioturbation intensity, shallow tiering, high sedimentation rate and/or event-bed deposition, significant levels of hydrodynamic energy, and low predation pressure. The studied ichnofauna reflects stressed environmental conditions, but it is unclear whether this reflects local brackish conditions (‘Gazpacho model’) or global hot temperatures (‘Hot Soup model’).

Givetian carbonate microfacies from the Totleben-2 well (Moesian Platform, central North Bulgaria)

Seven carbonate microfacies types (MFT 1–7) have been distinguished and described in the Givetian dolomite formation from the Totleben-2 well (Moesian Platform, central North Bulgaria). They have been grouped in three microfacies associations: 1) shallow subtidal microfacies with open circulation (MFT 1, coral-stromatoporoid floatstone/rudstone; and MFT 2, brachiopod floatstone/rudstone); 2) protected shallow subtidal microfacies (MFT 3, intraclastic-ooidal packstone/grainstone; MFT 4, palaeosiphonoclad wackestone/packstone; and MFT 5, bioclastic-peloidal packstone/grainstone); 3) intertidal-supratidal microfacies (MFT 6, fenestral microbial bindstone; and MFT 7, mudstone to packstone with Palaeomicrocodium). These microfacies have been interpreted as formed in dominantly shallow subtidal lagoon setting with open to moderate circulation and locally developed intertidal/supratidal environments. Most microfacies are comparable with Wilson’s (1975) Standard Microfacies Types (SMFT) and/or with Givetian microfacies described from the eastern part of the Moesian Platform, as well as with other shallow-marine Devonian successions from Europe.

Early Jurassic microbial mats—A potential response to reduced biotic activity in the aftermath of the end-Triassic mass extinction event

Wrinkle structures are microbially induced sedimentary structures (MISS) formed by cyanobacteria and are common in pre-Cambrian and Cambrian siltstones and sandstones but are otherwise rare in the Phanerozoic geological record. This paper reports the first discovery of Mesozoic wrinkle structures from Sweden. These are preserved in fine-grained and organic-rich heterolithic strata of the Lower Jurassic (Hettangian) Höganäs Formation in Skåne, southern Sweden. The strata formed in a low-energy, shallow subtidal setting in the marginal parts of the Danish rift-basin. Palynological analyses of fine-grained sandstones hosting the wrinkle structures show that the local terrestrial environment probably consisted of a wetland hosting ferns, cypress and the extinct conifer family Cheirolepidaceae. Palynostratigraphy indicates a Hettangian age, still within the floral recovery phase following the end-Triassic mass extinction event. The finding of wrinkle structures is significant as the presence of microbial mats in the shallow subtidal zone, (in a deeper setting compared to where modern epibenthic microbial mats grow) suggests decreased benthic biodiversity and suppressed grazing in shallow marine environments in the early aftermath of the end-Triassic mass extinction event.

A remarkable example of a Late Jurassic shallow-water ophiuroid assemblage from the Swiss Jura Mountains

Although much effort has been put into attempts to unravel the Mesozoic ophiuroid fossil record, surprisingly little attention has been paid to the geological history of family-level compositions of brittle star assemblages. Here, we describe new ophiuroid material from the Late Oxfordian Bure Member as exposed in a construction site along the federal Highway A16 (Transjurane Highway) near Boncourt–Queue au Loup (Swiss Jura Mountains). The remains are exceptionally well preserved and partially articulated, enabling detailed morphological observations at a level comparable to Recent material. Three species are identified; they are here shown to display unequivocal affinities with extant ophiuroid lineages. The assemblage comprises a new ophiacanthid genus and species, Juracantha hottingeri, which is closely related to extant Ophientrema, an ophiolepidid close to extant Ophiozonella described here as a new genus and species, Eozonella bergeri, and a species previously described as Ophiacantha? francojurassica, which is here reassigned to the extant ophionereidid genus Ophiodoris. The assemblage was found in association with Nanogyra oyster buildups encrusted by serpulids and associated with partially-articulated crinoids, deposited in a shallow subtidal setting. Its family level composition is highly unusual with respect to modern equivalents, and instead is reminiscent of modern bathyal assemblages.

Arabian carbonate reservoirs: A depositional model of the Arab-D reservoir in Khurais field, Saudi Arabia

The Upper Jurassic Arab Formation in the Arabian Peninsula, the most prolific oil-bearing interval of the world, is a succession of interbedded thick carbonates and evaporites that are defined stratigraphically upsection as the Arab-D, Arab-C, Arab-B, and Arab-A. The Arab-D reservoir is the main reservoir in Khurais field, one of the largest onshore oil fields of the Kingdom of Saudi Arabia. In Khurais field, the Arab-D reservoir is composed of the overlying evaporitic Arab-D Member of the Arab Formation and the underlying upper part of the Jubaila Formation. It contains 11 lithofacies, listed from deepest to shallowest: (1) hardground-capped skeletal wackestone and lime mudstone; (2) intraclast floatstone and rudstone; (3) pelletal wackestone and packstone; (4) stromatoporoid wackestone, packstone, and floatstone; (5) Cladocoropsis wackestone, packstone, and floatstone; (6) Clypeina and Thaumatoporella wackestone and packstone; (7) peloidal packstone and grainstone; (8) ooid grainstone; (9) crypt-microbial laminites; (10) evaporites; and (11) stratigraphically reoccurring dolomite. The Arab-D reservoir lithofacies succession represents shallowing-upward deposition, which, from deepest to shallowest, reflects the following depositional environments: offshore submarine turbidity fans (lithofacies 1 and 2); lower shoreface settings (lithofacies 3); stromatoporoid reef (lithofacies 4); lagoon (lithofacies 5 and 6); shallow subtidal settings (lithofacies 7 and 8); peritidal settings (lithofacies 9); and sabkhas and salinas (lithofacies 10). The depositional succession of the reservoir represents a prograding, shallow-marine, reef-rimmed carbonate shelf that was subjected to common storm abrasion, which triggered turbidites.

Mesoproterozoic silicified microbiotas of Russia and India's Characteristics and Contrasts

The paper analyses eight silicified Mesoproterozoic microbiotas of peritidal and shallow subtidal settings from Siberia, Ural and India. These microbiotas, subdivided into three main types - Kotuikan, Satka and Kataskin-are characterized by different taxonomic composition of microfossils. Mat-building entophysalidacean algae Eoentophysalis, ellipsoidal akinetes of nostocalean cyanobacteria genus Archaeoellipsoides and spherical large planktic microfossils Myxococcoides grandis of uncertain affinities dominate the Kotuikan-type microbiotas, the short trichomes are a rare but a distinctive element of these assemblages. The Satka type microbiotas are dominated by mat-building hormogonian cyanobacteria of genus Siphonophycus and chroococcacean dwellers genera Gloeodiniopsis, Eosynechococcus, Sphaerophycus, whereas entophysalidacean cyanobacteria are conspicuously missing and akinetes of genus Archaeoellipsoides occur but never abundant. Besides, microbiotas of Satka type include morphologically simple remains of phytoplanktic eukaryotic microorganisms-sphaeromorphic acritarchs genera Satka, Pterospermopsimorpha, Granomarginata? and Leiosphaeridia. The late Mesoproterozoic Kataskin-type microbiotas contain mat-forming entophysalidacean, oscillatoriacean and nostocalean as well as mat-dwelling and planktic chroococcacean cyanobacteria, but the most typical feature of these microfossil assemblages is the presence of a stalked cyanobacterium, Polybessurus bipartitus.
 Almost all-available data on relevant silicified Mesoproterozoic microbiotas from China, Greenland and North America have been analysed. Further different types of Mesoproterozoic silicified microbiotas have been compared with Palaeo- and Neoproterozoic microbiotas in cherts as well as with the assemblages of organic-walled microfossils throughout the world and explained differences and similarities in their composition. The analysis indicate that the Mesoproterozoic microbiotas have their own specific taxonomic composition and differ from the Palaeo- and Neoproterozoic microfossils occurring in the same and different palaeoenvironmental setting. The presence of newly evolved type of cyanobacteria, red algae and acanthomorphic acritarchs in the Kataskin-type microbiotas and contemporaneous open-shelf facies suggest that the terminal Mesoproterozoic can be separated as an independent biostratigraphical unit.

Oldest scleractinian coral reefs on the North American craton: Upper Triassic (Carnian), northeastern British Columbia, Canada

Bioclastic accumulations composed of crinoids, brachiopods, molluscs, spongiomorphs and scleractinian corals occur within Upper Triassic strata of the lower Baldonnel Formation at Pardonet Hill in northeastern British Columbia Canada. These small buildups (∼100 to 500 m 3) have planar bases and broadly convex tops. These mounds are interpreted as small patch reefs composed of packstone, bioclastic floatstone/rudstone and carbonate breccia intercalated with mixed siliciclastic carbonate sediments deposited in a shallow subtidal setting (i.e. above fairweather wave base). Amalgamated hummocky cross-stratified to current ripple-laminated, quartz-dominated sandstone beds and numerous sharp-based, normally graded bioclastic (commonly encrinitic) packstone/grainstone — quartz–sandstone couplets characterize inter-reef lithologies. Conodont biostratigraphy indicates that the Pardonet Hill patch reefs occur within strata dated as earliest Upper Carnian (lower nodosus zone). The Pardonet Hill patch reefs originated and developed during an interval of regional sea level lowstand. Strata within which these patch reefs occur represent the westernmost migration of the Triassic shoreline in western Canada. Disappearance of coral reefs in the study area may have been affected by rapid marine transgression and failure of reef faunas to recolonize the new shore zone further to the east. The Pardonet Hill locality occurred on the western margin of the North American craton during the Triassic. Prior to their discovery reef-like structures dominated by corals in the western Panthalassa were limited to allochthonous terranes (now part of the Cordillera). The Pardonet Hill patch reefs occur at approximately 30° Triassic paleolatitude. In modern settings, this is at the extreme latitudinal margin of subtropical zooxanthellate reef development. The presence of benthic faunas characteristic of low-paleolatitude settings on the northwestern coast of Pangea has significant implications in paleotectonic and paleoenvironmental reconstructions.

Infidelities of fossil assemblages

Infidelities of fossil assemblages

Lower Ordovician trilobites from the Baumann Fiord Formation, Ellesmere Island, Arctic Canada

Sections through member B of the Baumann Fiord Formation on the Bache Peninsula, Ellesmere Island, Arctic Canada, contain a range of lithofacies indicating deposition in shallow subtidal settings above storm wave base. Macrofossils are generally rare, but low-diversity trilobite-dominated faunas occur in a section near Sanddöla Creek. The only other fossils common in the collections are poorly preserved gastropods. All of the trilobite species are apparently new, and four are formally named: Licnocephala sanddoelaensis, "Peltabellia" baumannensis, Bolbocephalus ellesmerensis, and Ceratopeltis bachensis. The species Ceratopeltis forteyi is introduced for material previously described from North Greenland. Rare additional species of Licnocephala and Jeffersonia are reported in open nomenclature. The trilobites occur in three stratigraphically and lithologically distinct associations, with species diversity ranging from two to four. Closest comparisons of the trilobite species all indicate that member B should be assigned to the Tulean Stage of the Ibexian Series. Rananasus Cullison, 1944, is placed in synonymy of Bolbocephalus Whitfield, 1890.

Dolomitization of Middle Eocene carbonate rocks, Abu Roash area, Egypt

Dolomitization is the most striking diagenetic process affecting the late Middle Eocene shelly limestone beds at Abu Roash and its neighboring areas. Petrographic investigations reveal that these rocks represent shelly-wackstone to packstone microfacies, that they were deposited probably within a shallow subtidal setting. The rock matrix has been subjected to both selective and complete dolomitization and replaced by equigranular mosaics of fine-to medium-grained dolomite (crystalline planar-s dolomite texture). Meanwhile, the embedded shell fragments show high resistance to dolomitization and their original calcite composition is mostly preserved. The petrographic features of the dolomite texture suggest that these dolomites were formed under near surface and low temperature conditions.

Shallowing-Upward Carbonate Cycles in the Belingwe Greenstone Belt, Zimbabwe: A Record of Archean Sea-Level Oscillations

ABSTRACT Shallowing-upward carbonate cycles in the 2650 Ma Cheshire Formation, Belingwe greenstone belt (Zimbabwe), closely resemble their Proterozoic and Phanerozoic counterparts. The cycles form part of a karstified carbonate-ramp sequence that is overlain by, and grades basinward into, siliciclastic turbidites. A single section of 74 cycles (1.5 m average thickness) was studied in detail. Two basic cycle types are recognized, both with an asymmetric facies stacking pattern. One cycle type contains open marine, subtidal shale at the base. Shale is intercalated with storm-generated sandstone and grainstone beds that become more common and thicken upward, indicating progressive shallowing. Wave-rippled ooid-intraclast grainstone beds and bedsets overlie shale or form the base of the second cycle type. Grainstone formed at or above fair-weather wave base as shoreface sand sheets in an agitated, shallow subtidal setting. Microbial laminites constitute the top of both cycle types and are interpreted as peritidal deposits. In the upper part of the studied section, microbial boundstones with aragonite pseudomorphs are intercalated with, or overlie, laminites and formed in a supratidal environment. The vertical facies distribution within a cycle is indicative of rapid submergence followed by gradual shallowing of relative sea level. High-frequency eustatic sea-level changes are favored over an autocyclic mechanism and tectonically induced allocyclicity as the controlling mechanism for the cyclicity. Hierarchies of stratigraphic cyclicity occur on different scales and may be a result of the combined effects of several orders of sea-level oscillations. Cycle recurrence ratios correspond well to the Milankovitch frequencies calculated for the Late Archean, suggesting that orbital climatic forcing may have been in operation in Archean times.

Palaeobathymetric reconstruction of peritidal carbonates: Late Barremian, Urgonian, sequences of Provence (SE France)

Peritidal platform carbonates of Late Barremian age (Early Cretaceous) cropping out in the Marseille region (Provence, SE France) tend to be arranged stratigraphically in deepening-up or shallowing-up sequences. Seven facies types (lithofacies) were defined in the studied stratigraphic interval, ranging from supratidal to shallow subtidal settings. There is a good correspondence between the inferred relative water depth of facies and their average bed thickness. Cumulative thickness of facies types expresses quantitatively their palaeobathymetric range and provides an estimate of their average water depth. Calculations show that the palaeobathymetry of supratidal/intertidal facies types is from 0 to 40 cm, i.e. a pertaining to a palaeotidal (microtidal) range. The average palaeobathymetry of rudist facies, ascribed to subtidal settings, fluctuates between 40 cm and nearly 160 cm. Measuring a large number of beds belonging to both shallowing-up and deepening-up sequences, and averaging the results, appear relevant to minimise the effect of compaction, pressure solution and synsedimentary progressive or abrupt changes in accommodation, which are expected to modify bed thickness and its original palaeobathymetric significance. The method proposed here has a good potential for palaeobathymetric reconstruction of ancient peritidal carbonates.