Fair-weather Wave Research Articles

Hardgrounds of the Ordovician Baltic Paleobasin as a Distinct Type of Sedimentation Induced by Cyanobacterial Mats

The cold-water Volkhov and warm-water Keila hardgrounds of the Ordovician Baltic Basin represent a specialized product of cyanobacterial communities of the seafloor, another member in a group containing stromatolites and microbially induced sedimentary structures. The leading role of cyanobacterial communities in the development of hardgrounds is suggested by their macrofeatures, structure, and elemental composition revealed by SEM equipped with a microprobe microanalyzer. During the formation of the hardgrounds, the cyanobacterial films were of considerable thickness (up to 4–5 mm), but lacked vertical zonation of bacterial communities, thereby being at an intermediate stage between the mature biofilms and true mats. The soft substrate underlying hardgrounds was inhabited by an abundant infauna. Thanks to bioturbation, the cyanobacterial film was in places destroyed, which prevented the development of hardgrounds. In the warm Late Ordovician basin of northern Estonia, cyanobacterial films formed hardgrounds on the surface of ripple marks. Abundant peloids and warm pore waters facilitated further cementation and hardening of the substrate underneath the hardground before the burial of the ripple marks and the hardground. Cyanobacterial films appeared simultaneously over large areas of the seafloor due to settling of planktonic bacteria. The extracellular polymeric substances (EPSs) released by these bacteria were rapidly mineralized within a maximum of one season, apparently at a depth of 15–25 m, below the fair weather wave base. The formation of hardgrounds in bioherms occurred in a shallower environment and was patchy due to the alternation of living and dead cyanobacterial mats, mineralizing mats and resulting hardgrounds. Inhabitants of biohermal hardgrounds were producers of carbonate detritus, including of micritic size, over vast areas of the seafloor surrounding the bioherm.

Mineralogical and chemical composition of Ediacaran-Cambrian pelitic rocks of The Tamengo and Guaicurus formations, (Corumbá Group - MS, Brazil): Stratigraphic positioning and paleoenvironmental interpretations

The Tamengo and Guaicurus formations, upper units of the Corumbá Group, are placed at Ediacaran-Cambrian transition due to biostratigraphy and chemostratigraphy data. The Tamengo Formation is composed mainly of limestones with interbedded mudrocks (siliciclastic, carbonate and mixed siliciclastic-carbonate mudrocks) and record occurrences of macro-fossils Cloudina and Corumbella. The Guaicurus Formation is characterized as an extensive and homogeneous siliciclastic siltstone package. This paper focus in sedimentary, petrographic, mineral (XRD) and chemical (XRF) characterizations of mudrocks and siltstones in order to define stratigraphic positioning of the Tamengo and Guaicurus formations, and indicate depositional environment. To this purpose, three areas were studied in the vicinity of Corumbá (MS-Brazil): Laginha and Corcal quarries and an outcrop along MS-243 road. The rocks of the Tamengo Formation, under microscope, present fluid diffusion features as well as irregular laminations, calcite laminas as evaporate pseudomorphes and biogenic mats. In contrast, siltstones of the Guaicurus Formation are persistently laminated, well sorted and homogenous. Both mudrocks interbedded in carbonate layers and siltstones are composed of detrital quartz, mica and diagenetic clay minerals. Furthermore, the major oxides compositions are broadly similar, except for Na2O, higher in the Guaicurus Formation. The presence of carbonate rocks and mudrocks within the Tamengo Formation imply environment favorable to carbonate formation with episodically immature siliciclastic supply, from very fine-grained rocks of a nearby area, interrupting carbonate depositions. The Guaicurus Formation, distinctively, is a result of an uninterrupted immature siliciclastic supply from a nearby source area, broadly similar to the Tamengo Formation, and was deposited under a low energy setting in a high water level, below fair-weather wave. Petrographic features and diagenetic clay mineral paragenesis – illite + chlorite + smectite – within the Tamengo Formation indicate fluid circulation event. This event resulted in a complex diagenetic history with a possible retrograde diagenesis recorded in the mudrocks of the Tamengo Formation. The siltstones of the Guaicurus Formation present neither petrographic evidences of fluid diffusion nor presence of smectite along the paragenesis illite + chlorite, constraining the event in the Tamengo Formation. After these characterizations, the research revealed that the superior portion of Corcal quarry (L7) exposes rocks from the Tamengo Formation.

Onuphionella corusca sp. nov.: an early Cambrian-type agglutinated tube from Upper Ordovician strata of Morocco

Abstract A variety of unrelated organisms produce agglutinated tubes as dwelling structures, and the habit has persisted since the Neoproterozoic. Onuphionella , an agglutinated tube composed of mica flakes, has been found in lower Cambrian strata on several palaeocontinents. In this paper, a new species of the genus, Onuphionella corusca sp. nov., is described from the Sandbian (Upper Ordovician) First Bani Group of the Erfoud region, Morocco. The First Bani Group was formed in a storm-dominated shelf environment below fair-weather wave base. The agglutinated tubes are found in abundance on numerous bedding surfaces, possibly representing repeated colonization between storm events, and the tube maker must have been a major part of the local ecosystem. This is the first report of Onuphionella from Ordovician rocks and the first from Gondwana. Supplementary material: Data tables for major oxide and trace element results are available at https://doi.org/10.6084/m9.figshare.c.4283612

Shell concentrations analysis in the Lower Permian carbonate rocks (Khan Formation) in Central Iran (Kalmard area)

ABSTRACT Shell beds are sedimentary features that can potentially provide important palaeoenvironmental information. Stratigraphic, taphonomic, and paleontologic features frequency along with their occurrence in the shell beds in carbonate rocks of Khan Formation in Central Iran vary across lithofacies. In Khan Formation, four taphonomic and sedimentologic types of shell beds have been distinguished, which are fair weather wave concentrations (bryozoan dominated shells), storm wave concentrations (articulated brachiopod dominated shells), primary biogenic concentrations (fusulinid dominated shells), and proximal temoestite (including echinoderm dominated, gastropod dominated, bivalve dominated and disarticulated brachiopod dominated shells). Bryozoan shells were formed under strong hydrodynamic energy above fair weather base. The large articulated brachiopod shells are developed below the fair-weather wave-base ; Fusulinid shells formed in lagoon environment of homoclinal carbonate ramp which approve in situ concentration with no transportation. In echinoderm, gastropod and disarticulated bivalve and brachiopod shell beds, the occurrence of disarticulated and convex-up shells in packstone, sharp and erosional base, normal grading, parallel and hummocky lamination in the uppermost part, all indicate proximal tempestites in medium hydrodynamic force between the storm wave-base and fair-weather wave-base. These four types of shell concentrations are found in distinct positions within the depositional sequence of Khan Formation.

Late Pleistocene evolution of the mixed siliciclastic and carbonate southwestern New Caledonia continental shelf/lagoon

The main island of New Caledonia (NC) is surrounded by the second largest barrier reef in the world after the Australian Great Barrier Reef. The Upper Pleistocene evolution of this barrier reef–lagoon complex offers the opportunity to examine the response of a modern mixed tropical siliciclastic/carbonate system to relative sea-level changes and to better constrain the relative role of lagoon backfilling and paleo-drainage system in sediment transfer across the shelf. This paper mainly focuses on the results of chronostratigraphic and sedimentological interpretations of new seismic, bathymetric and coring data collected in the SW NC lagoon. Several significant points are highlighted from this study and allow the construction of a refined model which includes the evolution of both the SW NC mixed shelf and lagoon. New results show that the NC lagoon has not recorded a reciprocal sedimentation during glacial-interglacial sea-level cycles but a unique mixed regime. Strong contrasts are observed and these depend above all on the two lagoon domains: the outer coral plateau and the inner lagoon depression corresponding to a semi-closed proximal basin which is separated from the plateau by a basement ridge. In the depression, clastic sediments remain dominant from early flooding to highstand stages while carbonate sedimentation continued through time across the coral plateau. This spatial contrast between terrigenous and carbonate deposits was maintained throughout various sea level cycles. Thus, even if spatial segregation is also observed for other examples of mixed shelf environments, the SW NC shelf appears quite unique in comparison to the other mixed systems when considering conceptual models of reciprocal sedimentation. Moreover, the presence of a semi-enclosed lagoon depression allows the preservation of larger volume of terrigenous deposits. This semi-filled geometry of the lagoon is assumed to be the result of several factors: a significant subsidence that offers accommodation for aggrading deposits, evacuation through channels of sediments mobilized during storm events, the tidal currents, and fairweather wave dynamics. Lastly, the inherited morphologic and structures may explain the geomorphic evolution of the “escarpment margin” type of the SW NC rimmed shelf and its differentiation with the opposite SE NC ramp type shelf.

Refining the late Silurian sea-level history of the Prague Syncline—a case study based on the Přídolí GSSP (Czech Republic)

A 50-m-thick section in the Požary quarry, Prague Syncline (Czech Republic) spanning the upper Silurian (uppermost Ludlow and Přidoli) to the lowermost Devonian (Lochkovian) has been studied using sedimentological and physical stratigraphical (gamma-ray spectrometry) methods combined with conodont biostratigraphy. Conodont data demonstrate the presence of local conodont biozones: “Ozarkodina” crispa (uppermost Ludlow)—Zieglerodina zellmeri (base of Přidoli)—Zieglerodina ivochlupaci—Delotaxis detorta—“Ozarkodina” eosteinhornensis s.s.—Zieglerodina klonkensis—Icriodus hesperius-optima (lowermost Lochkovian). The studied section represents a transgressive–regressive facies succession characterized by a transition from distal calciturbidites deposited in a distally steepened carbonate platform during transgression and sea-level highstand (crispa to lower ivochlupaci zones) to mixed calciturbidites/coarse-grained bioclastic limestones (falling-stage systems tract, upper ivochlupaci to lower klonkensis zones). The upper part of the succession consists of bioclastic limestones corresponding to a subtidal setting between storm-wave and fair-weather wave base (lowstand systems tract in upper klonkensis and hesperius-optima zones). This interpretation corresponds to the late Silurian global sea-level pattern as recorded in other regions. Thus the depositional system is interpreted as mostly driven by eustasy with short-lived periods of influence from local tectonics.

FEEDING TRACES OF RECENT RAY FISH AND OCCURRENCES OF THE TRACE FOSSIL PISCICHNUS WAITEMATA FROM THE PLIOCENE OF SANTA MARIA ISLAND, AZORES (NORTHEAST ATLANTIC)

The bowl-shaped trace fossil Piscichnus waitemata Gregory 1991 appears in Pliocene sandstones from Santa Maria Island (Azores Archipelago), extensively excavated during a stage of island evolution when the volcanic edifice was a guyot (flat-topped seamount) isolated in the NE Atlantic. The host sediments were deposited at depths from the intertidal zone to fair-weather wave base in a tropical climate under the influence of periodic storms and hurricanes. The traces were produced by ray fishes hunting for polychaetes, crustaceans and bivalves living in the sediment, similar to present-day nearshore, warm waters in the Azores, Baja California Sur (Mexico), and New Zealand, from which examples of feeding depressions are drawn (incipient Piscichnus). While P. waitemata is abundantly present in planar sediments on top of the guyot, far fewer trace fossils occur in sandstone deposited on the guyot's margins. Presumably, the different densities of ray holes in the two sedimentary bodies were a response to lesser availability of prey, lower seawater temperatures (due to greater depths), and a more dynamic environment in which life conditions were less favorable. Moreover, the potential preservation of bowl-shaped depressions was lower in this setting, given the steepness of the seafloor, stronger currents, and constant sediment mobility. The top of the guyot was a more favorable habitat, refuge and/or nursery ground for many ray fishes. Measurement of the diameters of the ray holes indicate three distinct size classes, which may suggest that several species were responsible for their formation.

Shell concentration dynamics driven by wave motion in flume experiments: Insights for coquina facies from lake-margin settings

Coquinas are important carbonate oil reservoirs in the sin-rift sequence of the Brazilian marginal basins, mainly in Campos and Santos basins. They are generated from shell accumulations in lake-margin environments in which waves and bottom currents, at storm conditions, were the main hydrodynamic agents of transport and reworking. In order to investigate the role of wave transformation processes in shallow waters – swash, breaking and wave shoaling – in shell concentrations, a physical modelling of the coastal environment was performed in laboratory. The experimental setup consisted of a 2D flume, a wave generator, and a sedimentary bottom composed by quartz sand and shells (bivalves, gastropods, and fragments). An underwater camera, wave gauges and an ADV were used to record the sedimentary processes and to perform a quantitative analysis of the hydrodynamics. The experimental scenario reproduced a rift-lake margin. A coastal sector of Lake Tanganyika, Tanzania, Africa, was used to set the model boundary conditions (in a 1/20 scale) as well as the coastal gradient and wave conditions. From 54 h of wave experiments which alternated from fair-weather to storm wave conditions, three shell concentration dynamics were observed: reworking, winnowing, and dynamic bypassing. The reworking (erosion) was characterized by traction, rolling, and saltation of bioclasts, together with the traction and suspension of sand. The occurrence of the latter was restricted to the breaking and swash zone in both fair-weather and storm conditions. The winnowing (erosion) was featured by sand suspension and the gentle traction and rotation of the bioclasts on the bottom. The winnowing occurred at the proximal area of the shoaling zone, just behind the first wave-breaking point in storm wave conditions. The dynamic bypassing (omission) was identified by the ripples migration over static shells on the bottom of the distal shoaling zone, which produced alternated burial and exhumation of bioclasts. The quantitative analysis of the oscillatory flow indicated higher velocities at the breaking zone (reworking domain) than at the shoaling zone (winnowing domain) mainly during storm conditions. This study provided new and more accurate insights to depositional models of lake-margin coquinas, suggesting that each wave transformation process control a specific dynamic of shell concentration. Also, the limit between the breaking zone and the shoaling zone in storm conditions (1st wave-breaking point), was identified as a better criterion to bound the reworking domain, where fragmented coquinas are produced, from the winnowing and dynamic bypassing domains, where non-fragmented coquinas occur, in contrast to the fair-weather wave base criterion.

A conceptual phosphogenesis model for the Red Sea phosphorites, Quseir area, Egypt

Along the Red Sea coast (at Quseir area), the Upper Cretaceous-Lower Paleogene phosphorite-bearing sediments were laid down under shallow marine conditions. There, thick economic phosphorite beds are intercalating the Quseir, Duwi, and Dakhla formations. Furthermore, thin phosphorite beds (< 20-cm thick) have been recorded as interbeds within the above-mentioned rock units. Each high-grade, economic phosphorite bed experienced many successive stages of phosphogenesis. The initial stage of phosphogenesis started with scattered phosphate peloids, which accumulated below fair-weather wave base and under calm conditions. Subsequently, the frequency and quantities of these peloids increased with time. The closing session of the phosphogenesis scenario was characterized by exhumation, fragmentation, and redeposition of the peloids by high-energy episodes (i.e., storms). Prolonged action of these high-energy events resulted in mixing of peloids with other phosphatic components in crudely fining upward sequences. Both one-event and amalgamated (multi-event) storm beds are common. The SEM investigations have indicated that algal blooms played a vital role in the origin of the Red Sea phosphorites. These algal blooms acted as important sites for P fixation and consequently, a release of P. Integrated field and laboratory studies have revealed that phosphorite rocks display noticeable variations from one locality to another. These variations include changes in thickness, P2O5 content and the hosting sediments (siliciclastics and/or carbonates). A plausible explanation for these variations may be related to the effect of synsedimentary local tectonics.

A CARBONATE RAMP EVOLUTION IN THE TRANSITION FROM THE APULIA PLATFORM TO THE IONIAN BASIN DURING EARLY TO LATE CRETACEOUS (NW GREECE)

Facies analysis of Cretaceous carbonate sequences from the external and central Ionian zone revealed a homoclinal ramp model of evolution. During Berriasian to Valanginian, the carbonate ramp was differentiated to an inner-mid and outer ramp environment, which corresponded to the external and central Ionian zone, respectively, while the main inner ramp environment is assumed that was located in the Pre-Apulian zone. The external Ionian zone (inner-mid ramp) is characterized by muds tones-wackes tones with fragmented echinoderms, bivalves, radiolarians and rare aptychus considered to be deposited below the fairweather wave base (FWWB). Locally, thin graded storm deposits intervene, indicating deposition above the storm weather wave base (SWB). Minor occurrences of packs tonesgrainstones, with fragmented echinoderms, calcareous algae, tubiphytes, lagenid foraminifera and rare ooids occur, as well, considered to be deposited at the lowermost part of the inner ramp, near the constantly agitated fairweather wave base (FWWB). The central Ionian zone (outer ramp) is mainly characterized by mudstones-wackestones with abundant radiolarians and rare calpionellids and calcispheres, considered to be deposited below the storm wave base (SWB). No talus or breccias deposits were observed, during the mentioned time interval, in any part of the studied area. From Hauterivian to Turonian, continual sea-level rise, led to establishment of outer ramp environment in the external Ionian zone, over the previous inner-mid ramp, and outer ramp-basin environment, over the previous outer ramp, in the central Ionian zone. The transition from shallower to deeper conditions is characterized by an overall deposition of mudstones-wackestones with abundant radiolarians rooted in pure micrite.

Ichnology and depositional environments of the Upper Ordovician Stony Mountain Formation in the Williston Basin, Canada: Refining ichnofacies and ichnofabric models for Epeiric Sea carbonates

Ordovician epeiric sea carbonates in intracratonic basins of Laurentia are enigmatic due to their unique depositional settings and the absence of modern analogs. Integrated ichnologic and sedimentologic analysis of the Upper Ordovician Stony Mountain Formation in the Williston Basin of Canada allows recognition of neritic-marine, nearshore-marine, open-lagoon, restricted-lagoon, peritidal sand-shoal and peritidal-flat subenvironments. The Cruziana ichnofacies occurs in neritic (between fair-weather and storm wave bases) and nearshore (around fair-weather wave base) marine environments. The depauperate Cruziana ichnofacies is present in open- and restricted-lagoon environments, indicating a shift from fully marine to stressed conditions. In the open lagoon, composite ichnofabrics related to omission surfaces illustrate the low rates of background sedimentation interrupted by event deposition and early cementation, illustrating the Glossifungites and Trypanites ichnofacies. The decreased size of discrete burrows in the restricted lagoon is attributed to reduced oxygenation under stagnation rather than hypersalinity. The peritidal complex includes high-energy sand shoals and low-energy tidal flats. Only monospecific colonization took place sporadically in associated subtidal environments within the peritidal complex deposits.

Trace fossils, sedimentary facies and parasequence architecture from the Lower Cretaceous Mulichinco Formation of Argentina: The role of fair-weather waves in shoreface deposits

Shorefaces can display strong facies variability and integration of sedimentology and ichnology provides a high-resolution model to identify variations among strongly storm-dominated (high energy), moderately storm-affected (intermediate energy), and weakly storm-affected (low energy) shoreface deposits. In addition, ichnology has proved to be of help to delineate parasequences as trace-fossil associations are excellent indicators of environmental conditions which typically change along the depositional profile. Shallow-marine deposits and associated ichnofaunas from the Mulichinco Formation (Valanginian, Lower Cretaceous) in Puerta Curaco, Neuquén Basin, western Argentina, were analyzed to evaluate stress factors on shoreface benthos and parasequence architecture.During storm-dominated conditions, the Skolithos Ichnofacies prevails within the offshore transition and lower shoreface represented by assemblages dominated by Thalassinoides isp. and Ophiomorpha irregulaire. Under weakly storm-affected conditions, the Cruziana Ichnofacies is recognized, characterized by assemblages dominated by Thalassinoides isp. and Gyrochorte comosa in the offshore transition, and by Gyrochorte comosa within the lower shoreface. Storm-influenced conditions yield wider ichnologic variability, showing elements of both ichnofacies.Storm influence on sedimentation is affected by both allogenic (e.g. tectonic subsidence, sea-level, and sediment influx) and autogenic (e.g. hydrodynamic) controls at both parasequence and intra-parasequence scales. Four distinct types of parasequences were recognized, strongly storm-dominated, moderately storm-affected, moderately storm-affected – strongly fair-weather reworked, and weakly storm-affected, categorized based on parasequence architectural variability derived from varying degrees of storm and fair-weather wave influence. The new type of shoreface described here, the moderately storm-affected – strongly fair-weather reworked shoreface, features storm deposits reworked thoroughly by fair-weather waves. During fair-weather wave reworking, elements of the Cruziana Ichnofacies are overprinted upon relict elements of the Skolithos Ichnofacies from previous storm induced deposition. This type of shoreface, commonly overlooked in past literature, expands our understanding of the sedimentary dynamics and stratigraphic architecture in a shoreface susceptible to various parasequence and intra-parasequence scale degrees of storm and fair-weather wave influence.

Coniacian sandstones from the North Sudetic Synclinorium revisited: palaeoenvironmental and palaeogeographical reconstructions based on trace fossil analysis and associated body fossils

AbstractThe Coniacian quartz sandstones (Żerkowice Member, Rakowice Wielkie Formation) that crop out at quarries near Czaple-Nowa Wieś Grodziska (North Sudetic Synclinorium) contain a low-diversity assemblage of trace fossils:Gyrochorteisp.,Ophiomorpha nodosaLundgren, 1891,Ophiomorphaisp.,Phycodescf.curvipalmatum(Pollard, 1981), ?Phycodesisp.,Planolitescf.beverleyensis(Billings, 1862),Thalassinoides paradoxicusWoodward, 1830 and ?Thalassinoidesisp. Moreover, interesting compound burrow systems, here referred to asThalassinoides-Phycodescf.palmatusand ?Thalassinoides-Phycodes, were recognised at the Czaple Quarry. Additionally, ?Gyrochorteisp.,Phycodescf.flabellum(Miller and Dyer, 1878) and ?Treptichnusisp. were encountered at correlative levels in the Rakowice Małe Quarry. Some of these ichnotaxa have not been recorded previously from Coniacian sandstones of the Żerkowice Member. Additionally, in slabs of these sandstones, the gastropodNerinea bicinctaBronn, 1836 and the bivalveLima haidingeriZittel, 1866 were found. These interesting finds, in particular the gastropods, were already noted from the study area in the first half of the twentieth century by Scupin (1912–1913). Ethologically, the trace fossil assemblage is represented by domichnia or domichnia/fodinichnia (Ophiomorpha,Thalassinoides), fodinichnia (Phycodes) and pascichnia (Gyrochorte,Planolites). The compound burrow systems (Thalassinoides-Phycodes) are interpreted as dwelling/feeding structures. The possible tracemakers are crustaceans (Ophiomorpha,Thalassinoides) or worm-like animals (annelids and other) (Planolites, ?Phycodes,Gyrochorteand ?Treptichnus). The assemblage of trace fossils is characteristic of theSkolithosichnofacies andCruzianaichnofacies, typical of shallow-marine settings. Ichnological studies, as well as the presence of accompanying fossils (bivalves, gastropods), confirm the palaeoenvironmental reconstruction of the Żerkowice Member sandstones by Leszczyński (2010). That author interpreted the Coniacian sandstones as bar and storm deposits laid down in a shallow epicontinental sea (mainly the foreshore-upper shoreface; up to the middle shoreface) under normal oxygenation and salinity, in soft substrate, above fair-weather wave base. The deposition of the Żerkowice Member sandstones is linked to a regression that started after uplift of the southeastern part of the North Sudetic Synclinorium.

Facies and depositional settings of the Middle Eocene-Oligocene carbonates in Kutch

Middle Eocene Fulra Limestone and Oligocene Maniyara Fort Formation represent platform carbonate deposits of Kutch at the north-western margin of India. These carbonates contain larger benthic foraminifera, including Alveolina, Assilina, Discocyclina, Lepidocyclina, Miogypsina, Nummulites and Spiroclypeus. This study presents paleodepositional and paleobathymetric interpretations for both formations using benthic foraminifera in combination with lithological association, sedimentary structures and early diagenetic features. The six carbonate facies comprising the Fulra Limestone indicate a depositional spectrum ranging from bar-lagoon to mid-ramp depositional conditions. It records several shallowing upward cycles, leading to emergence and formation of paleokarst. The four carbonate facies of the Maniyara Fort Formation represents deposition within the inner ramp setting in bar-lagoon and patch-reef environment, while intervening fine siliciclastics correspond to episodes of relative sea level fall. Nummulitic accumulations form low-relief bars within the fair weather wave base in both the formations. The depositional setting of the Paleogene carbonate in Kutch broadly resembles Eocene platformal deposits in the circum-Tethys belt.

Sedimentology in metamorphic rocks, the Willyama Supergroup, Broken Hill, Australia

ABSTRACTIn the upper greenschist to granulite grade rocks of the Willyama Supergroup at Broken Hill, Australia, earlier recognition of metamorphosed graded bedding in siliciclastic metasedimentary rocks led to interpretations of these rocks as deep-water turbidites. However, graded beds can also be deposited in shelfal environments below storm wave base. This study identified other tempestite features including wave oscillation ripples, hummocky cross-stratification and swaley cross-stratification indicating that deposition took place above the wave base of the larger storms.Albitised metasedimentary rocks of the upper Thackaringa Group show structures such as swaley cross-stratification typical of shallow-water conditions above fair-weather wave base. Deposition of the Broken Hill Group commenced with muddy Allendale Metasediments conformable on the Thackaringa Group. The Ettlewood Calc-Silicate Member, originally a dolomitic, siliceous sediment, is interpreted as coastal sabkha indicating onset of a marine transgression. The Parnell Gneiss represents a volcanic or volcaniclastic interruption, heralding gradually increasing input of sand in the Freyers Metasediments reaching a maximum in middle Freyers Metasediments, followed by an abrupt reversion to mud, still influenced by wave action. An open marine shelf is interpreted, possibly 30 m deep (no more than 100 m) in the final stage of a developing rift. The Broken Hill Group terminated with the massive Hores Gneiss volcanic unit.Sedimentation of the siliciclastic Sundown Group took place in similar conditions, commencing with a muddy interval overlying the Hores Gneiss. The shallowing produced by ∼90 m thickness of volcanic/volcaniclastic Hores Gneiss was compensated by subsidence.Paragon Group deposition commenced with substantial black mud, resulting from isolation from the sand supply and probably isolation from the sea. A fresh connection to the sea led first to the deposition of dolomitic carbonate (King Gunnia Calc-Silicate Member), then to deposition of parallel-laminated fine sand below wave base (upper Cartwrights Creek Metasediments), followed by ripple cross-laminated sand above wave base (Bijerkerno Metasediments). The Dalnit Bore Metasediments show abundant very thin graded silt–mud units possibly deposited below storm wave base, and thicker units of stacked wave oscillation ripples deposited above the wave base of larger storms.The Broken Hill orebody is hosted by altered Broken Hill Group metasedimentary rocks deposited at water depths of ∼30 m. Unless the ore fluid temperature was less than 150°C, it is likely that the orebody formed below the seafloor: at such shallow-water depths, the confining pressure would be inadequate to suppress boiling of hotter rising hydrothermal fluids.

Microfacies and Depositional Environments of Miocene Isolated Carbonate Platforms from Central Luconia, Offshore Sarawak, Malaysia

AbstractThe Luconia Province – offshore Sarawak – is a key geological unit for understanding the distribution of hydrocarbon resources in Malaysia. Nevertheless, little effort has been made to address the palaeoenvironmental characteristics of the Tertiary carbonates in the key sector of Central Luconia. We study the sedimentology and petrography of core samples from a well in Central Luconia, for which thirteen microfacies have been identified reflecting different depositional settings. This is the first microfacies scheme elaborated for Luconian carbonates. Lithofacies and microfacies distribution are compatible with deposition in a reef complex, originating around a framework reef, within the euphotic zone. Sediments were deposited in environments of backreef, reef crest, and forereef. The fair weather wave base is marked by the presence of coralline red algae, foraminifera, decreasing degree of bioclast fragmentation and other microfacies features. As a result, a depositional‐environmental model is constructed, depicting a reef complex built around a framework reef developed on the margin of an isolated platform. In addition, an innovative, preliminary time series analysis of facies, microfacies and depositional environment data reveal the existence of seasonal cycles in the stacking patterns of facies and microfacies.

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.

Traços fósseis dos depósitos marinhos rasos da Formação Pitinga, Siluriano superior da Bacia do Amazonas, Rio Tapajós, PA, Brasil

The Silurian-Devonian interval of the Amazonas Basin exhibits low trace fossils diversity. Transgressive post-glacial deposits (upper Silurian) in the Tapajos River edge shows a trace fossils assemblage composed by Arthrophycus, Beaconites, Bifungites, Diplichnites, Lockeia, Monocraterion, Planolites and Phycodes, also indeterminate tubes and indeterminate arthropod scratches. This assemblage illustrates the Cruziana Ichnofacies preserved in laminated siltstone, wavy bedding sandstone with tabular and sigmoidal cross-stratification. The integration of ichnologic and sedimentologic datasets suggests a wave-dominated shallow marine palaeoenvironment. The predominance of epifaunal trace fossils indicates relatively calm water and deposition just below of the fair-weather wave base. The tracemakers were mostly worms and arthropods, but also rare bivalves. The gradual retreat of glaciers in the Silurian–Devonian transition led to the eustatic rise and the emergence of news ecospace in transgressive post-glacial sea. Thereby, the low icnofaunistic diversity could be related to cold seas that dominated the Gondwana coast enabling infaunal occupation by organisms adapted to the environment stress during the Silurian. Oscillations in the sedimentation rates and erosion of the shallow-tiers traces fossils may also be responsible by the low diversity recorded in the studied deposits.

Microfacies and depositional environment of Late Cretaceous to Early Paleocene oil shales from Jordan

The purpose of this study is to better understand the depositional setting of Late Cretaceous to Early Paleocene oil shales from southeast and central-east Jordan. One core from both the Jafr and the Azraq-Hamza Basins was logged, and their lithology, texture, and ichnofabrics were recorded. A total of 79 thin sections were analyzed petrographically, and eight microfacies types recognized. Both cores show lithologic and petrographic similarities. The oil shales are Maastrichtian to Danian in age and can be described as organic matter and calcite-rich mudrocks. The most abundant granular components are foraminifera and various types of phosphatic bio- and lithoclasts. Macrofossils (bivalves, ostracods, echinoderms) were recorded in some intervals. The current results were compared with data from a previous publication on Maastrichtian oil shales from the Jafr Basin. A new model explaining the deposition of the oil shales of the Jafr and Azraq-Hamza Basins is proposed. The onset of the Maastrichtian oil shale deposition in both basins coincides with the early Maastrichtian transgression in this region. The organic matter-rich sediments were deposited in a mid to outer ramp setting below the storm wave base. Younger oil shales of Late Maastrichtian to Danian age were deposited in a shallower environment, below the fair weather wave base. The Cretaceous/Paleogene boundary is marked by a hiatus in both cores. The Danian oil shales show relatively lower “total organic carbon” content than the Maastrichtian ones. The former are believed to have been deposited in more oxygenated bottom waters of a mid-ramp zone.

PREFERENTIAL ORIENTATION OF SHRIMP-GENERATED DIPLOCRATERION PARALLELUM AND THEIR RELIABILITY AS PALEOCURRENT INDICATORS

Arthropods from the late Campanian (Late Cretaceous) of the Western Interior Seaway produced U-shaped Diplocraterion parallelum in mudstones along two closely spaced surfaces (10 cm apart), one of which corresponds to a maximum transgressive surface. Diplocraterion parallelum are widely distributed across both surfaces, with substantial variations in burrow orientations. Based on a comparison of paleocurrent indicators to burrow orientations, we demonstrate that D. parallelum are preferentially oriented parallel to the prevailing fair-weather wave propagation direction (wave-forced currents) that acted upon the colonized surfaces. There is an apparent maximum (34% above uniformity) preferential orientation of the burrows, attributed to the fact that wave-forced currents represent only one of several factors controlling shrimp burrowing behaviors. Based on this study, we propose that in the absence of paleocurrent data, Diplocraterion and other U-shaped burrows can be used to resolve flow directions, and that preferential burrow orientations ≥ 2% above uniformity are significant. However, it is noted that it is only possible to resolve 2-way flow directions (trend) from U-shaped burrows, as there is no way to determine vector directions.