Mixed Carbonate-siliciclastic Shelf Research Articles

Facies character and evolution of a mixed carbonate–siliciclastic shelf: Upper Triassic–Lower Jurassic succession in the eastern Northern Calcareous Alps (Stumpfmauer, Austria)

Western Tethys sedimentary successions constitute fundamental archives of Late Triassic–Early Jurassic environmental, carbonate production and tectonic changes. During the Late Triassic, the Northern Calcareous Alps (Austria) belonged to the Western Tethys passive margin, characterised by the deposition of the early-dolomitized peritidal Hauptdolomit (Norian) adjacent basinward to the Dachstein carbonate shelf and passing upward to the mixed carbonate–siliciclastic Kössen Formation (Upper Norian–Rhaetian). The Kössen Fm. was subdivided into the lower shallow-water Hochalm Member and the upper Eiberg Member, accumulated in an intraplatform basin coeval to shallow-water carbonates (Upper Rhaetian Limestone). The Eiberg Mb. and overlying Jurassic strata were extensively studied as a continuous marine record across the Triassic/Jurassic boundary. In contrast, shallow-water successions, time-equivalent to the Eiberg Mb. and Upper Rhaetian Limestone, located North of the Eiberg Basin, are poorly investigated. This study focusses on the approximately 350 m thick Norian to Lower Jurassic succession cropping out in the eastern Northern Calcareous Alps (Stumpfmauer). The 32 distinguished lithofacies compose seven, vertically superimposed, sedimentary units (A–G), belonging to the Hochalm Mb. of the Kössen Fm. (Unit A peritidal cyclothems, Unit B claystone/marlstone with fossiliferous beds, Unit C coral boundstone to floatstone), Upper Rhaetian Limestone (Unit D subtidal cyclothems with claystone), shallow-water carbonate strata transitional to Lower Jurassic (Unit E ooidal coated grain peloidal grainstone with basal transgressive lag, Unit F bivalve-rich, microbialite and oncoidal lithofacies, previously attributed to the Upper Rhaetian Limestone) and Hettangian–Sinemurian Kalksburg Fm. (Unit G cross-laminated coated grain peloidal grainstone with quartz and chert). The detailed lithofacies characterisation presented in this study contributes to the knowledge on the Northern Calcareous Alps stratigraphy and depositional environments across the Triassic/Jurassic boundary. The identified sedimentary units can be framed in the evolution of Western Tethys and share similarities with depositional systems from the Western Carpathians, Transdanubian Range, Southern Alps and Dinarides suggesting coherent sedimentary response and environmental, climate and tectonic controls in different palaeogeographic domains.

Orbital forcing of late Early Devonian storm events on a mixed carbonate-siliciclastic shelf, Longmenshan area, Sichuan Province, China

AbstractThe Yangmaba Formation of latest Early Devonian age in the Longmenshan area of Sichuan Province, China, is a shelfal facies that consists of four types of carbonate-siliciclastic deposits: clay-rich, siliciclastic sand-rich, carbonate-dominated, and hybrid mixed. Storm deposits vary in their thickness, composition, and abundance-per-meter within these mixed carbonate-siliciclastic deposits. Meter-scale statistics of the relative storm frequency (events-per-meter) and magnitude (bed thickness) were compiled in two coeval sections through each of the hybrid facies in the lower Yangmaba Formation, which spans approximately the entire conodont Polygnathus patulus Zone of the uppermost Emsian Stage. Low-pass filtering, spectral analysis, and Acycle software interpretation of these tempestite statistics yield 3.5 main oscillations in each section, with an average wavelength of ~23 m. These long-wavelength trends are semi-coincident with interpreted long-term variations in sea level, where shallower depths allowed a greater influence by storms. Superimposed on the long-wavelength cycles are medium-wavelength cycles of 5.5–6.3 m. The estimated ~1.5 m.y. time-span of this conodont zone of the lower Yangmaba Formation and the approximate 1:4 ratios of these wavelengths indicate that frequency and intensity of major storms and the recording of tempestites in the sedimentary record were modulated by ~100- and 95-k.y.-short-eccentricity orbital-climate oscillations superimposed on a main 405-k.y.-long-eccentricity cycle. These eccentricity climate cycles governed storm intensity and regional sea level on this margin of the tropical Yangtze Platform of South China.

Carbonate-shelf evolution during the Oligocene to early Miocene: insights from shelf architecture, lithofacies, and depositional models of the Kujung Formation, offshore East Java, Indonesia

ABSTRACTOligocene–Miocene carbonates are prolific hydrocarbon reservoirs in Southeast Asia. Extensive subsurface data for this stratigraphic section has become available through exploration and production activities. A carbonate shelf in the study area showed an evolution in shelf architecture and lithofacies during this period. Despite the economic importance, complexity, and data availability, there have been few published studies on a comprehensive description and interpretation of carbonate-shelf evolution in the region.We utilized data from nineteen wells and 1,300 km2 of 3D seismic data from offshore East Java to study the Oligocene–Miocene Kujung Formation. An average of 700 m stratigraphic sections from this formation were analyzed. This interval spans 13.5 million years (My) from the Rupelian, Chattian, through Aquitanian constrained by numerical ages from 87Sr/86Sr. The Kujung Formation is subdivided into the Rupelian–Chattian mixed-siliciclastic–carbonate shelf (MSCS) and the Aquitanian carbonate-buildups shelf (CBS) based on shelf architecture and lithofacies. The boundary between the MSCS and CBS is interpreted to be near the Oligocene–Miocene boundary at 23 Ma. Accumulation rates in the CBS are up to three times greater than that of the MSCS. We propose new depositional models for the Kujung Formation, which was used to discuss the dominant controls on shelf evolution.This study suggests that although climate played a role in dictating environmental conditions during the Oligocene–Miocene, the dominant factors controlling carbonate-shelf evolution in the study area appear to have been antecedent topography, routing of siliciclastic sediment, and patterns of sea-level fluctuation.

First record of trace fossils from the Oxfordian Argiles rouges de Kheneg Formation (Tiaret, northwestern Algeria)

Three main facies associations FA-3 to FA occur in the Oxfordian Argiles rouges de Kheneg Formation in northwestern Algeria. They correspond respectively to the deeper part of a mixed siliciclastic-carbonate shelf, upper shoreface and offshore transition-lower offshore. The trace fossil association of the Argiles rouges de Kheneg Formation contains fifteen ichnogenera and is moderately diverse for the Upper Jurassic. The formation contains diverse and abundant deep water or dominantly deep water trace fossils (i.e. Belorhaphe, Chondrites, Helminthopsis, Nereites, Megagrapton). They indicate that a part of the formation was deposited in offshore transition to lower offshore environments.

Periplatform ooze in a mixed siliciclastic-carbonate system - Vaca Muerta Formation, Argentina

Thick carbonate mud successions and marl/limestone alternations are often found in periplatform environments. The sources of these fine-grained sediments and the processes that govern their accumulation and distribution in the basin are frequently a matter of debate. This case study from a mixed siliciclastic-carbonate succession in the Neuquén Basin, Argentina, relates the origin of the carbonate-dominated intervals to shelfal carbonate production. The Tithonian-early Valanginian basinal strata (Vaca Muerta Formation) contains several carbonate-rich intervals at different stratigraphic levels. The thickest of these intervals is the mid-late Tithonian Los Catutos Member in Sierra de la Vaca Muerta. This wedge-shaped package of micritic limestones and intercalated marls thins basinwards. Proximal-distal variations and cyclic partitioning in facies and geochemical proxies indicate highstand shedding and a periplatform origin for the Los Catutos Member. The proposed source is a coeval 20 m-thick pure carbonate interval located 50 km south of the Sierra de la Vaca Muerta within the mixed siliciclastic-carbonate shelf facies of the Picún Leufַú Formation. Younger carbonate-rich intervals in the center of the basin (Puerta Curaco area) show similar characteristics to the distal portion of the Los Catutos Member and are also interpreted as periplatform deposits but sourced from carbonate shelves farther east. The high sedimentation rates of the Los Catutos Member require an efficient mechanism, other than tides and winds, to deliver significant amounts of fine-grained carbonate sediment to the basin. This mechanism is envisioned to be sediment-charged hyperpycnal flows cascading from the platform to the basin, analogous to what is observed in modern carbonate systems. The occurrence of periplatform ooze in different stratigraphic levels of the siliciclastic-dominated Vaca Muerta Formation demonstrates that off-shelf transport and distribution of carbonate mud in the basin are also common processes in mixed clastic-carbonate environments.

Bottomset and foreset sedimentary processes in the mixed carbonate-siliciclastic Upper Jurassic-Lower Cretaceous Vaca Muerta Formation, Picún Leufú Area, Argentina

The Upper Jurassic-Lower Cretaceous Vaca Muerta Formation at the Picún Leufú area constitutes bottomset and foreset marine deposits comprising open bay, siliciclastic basin and mixed carbonate-siliciclastic shelf facies. Detailed sedimentological, ichnological and sequence stratigraphic analysis of six stratigraphic sections allows establishing two depositional sequences. Depositional Sequence 1 (DS1) begins with lowstand eolian deposits included in the Quebrada del Sapo Formation, and continues with thin, retrograding, transgressive open bay facies of the Vaca Muerta Formation encompassing coastal sand sheet and dune, marginal marine, bay margin and distal bay facies. Above the transgressive deposits, the regressive hemicycle of DS1 consists of bottomset deposits representing a siliciclastic basin facies association. Lobe and lobe fringe hyperpycnal flow sedimentation alternated with hemipelagic basinal deposition in an oxygen-deficient environment. A relative sea-level fall generated a sequence boundary, which coincides with an angular unconformity that marks the base of Depositional Sequence 2 (DS2). This sea-level fall triggered the formation of an extensive lowstand channel-fill and lobe complex at the base of the slope, followed by retrogradational lobe facies during the subsequent transgression. The regressive hemicycle of DS2 represents foreset mixed carbonate-siliciclastic shelf facies, forming a slope mud belt and slope sand bodies. Near the top, sedimentation in a foreset-topset transition resulted in the accumulation of bioturbated mixed slope and sandy shoal deposits. Sand bars and lagoonal facies occur on top of all sections and are truncated by a sequence boundary. The combined analysis provides insights into the sedimentary processes affecting bottomset and foreset deposition, underscoring the role of wave-influenced hyperpycnal flows, and the effect on organic matter dilution in one of the most important unconventional reservoirs from Argentina.

Neogene evolution and demise of the Amapá carbonate platform, Amazon continental margin, Brazil

The Amazon continental shelf hosted one of the world's largest mixed carbonate-siliciclastic platforms from the late Paleocene onwards - the Amapá carbonates. The platform architecture, however, remains poorly understood and causes and timing of the cessation of carbonate deposition are still controversial. Here we present a stratigraphic analysis of the Neogene succession of the Amapá carbonates, based on a grid of 2D/3D seismic data correlated to revised micropaleontological data from exploration wells. The results provide improved constraints on the age of the transition from predominantly carbonate to siliciclastic sedimentation, which is shown to have varied through time across three different sectors of the shelf (NW, Central and SE). Four Neogene evolutionary stages of carbonate deposition could be defined and dated with reference to the new age model: (1) between ca. 24 and 8 Ma a predominantly aggrading mixed carbonate-siliciclastic shelf prevailed across the entire region carbonate production gave way to siliciclastic sedimentation across the Central and SE shelves; (2) between 8 and 5.5 Ma carbonate production continued to dominate the NW shelf, as deposition was able to keep up with base level oscillations; (3) between 5.5 and 3.7 Ma (early Pliocene), sediment supply from the paleo-Amazon River promoted the progressive burial of carbonates on the inner NW shelf, while carbonates production continued on the outer shelf (until 3.7 Ma). Longer-lasting carbonate sedimentation on the NW shelf can be explained by a lesser influx of siliciclastic sediments due to the paleo-geography of the Central shelf, characterized by a 150-km-wide embayment, which directed most terrigenous sediments sourced from the paleo-Amazon River to the continental slope and deep ocean; (4) from 3.7 Ma onwards, when the Central shelf embayment became completely filled, continuous sediment supply to the NW shelf resulted in the final transition from carbonate to siliciclastic-dominated environments on the entire Offshore Amazon Basin.

Lateral variability of shallow-water facies and high-frequency cycles in foreland basin carbonate platforms (Pennsylvanian, NW Spain)

The kilometer-sized and 100-meter-thick carbonate platforms of the Escalada Fm. I and II (Middle Pennsylvanian) accumulated in the foredeep of a marine foreland basin during the transgressive phases of 3rd-order sequences and were buried by prograding siliciclastic deltaic systems in the course of the subsequent highstand. The carbonate successions show a general upward trend from grain- to mud-supported carbonates, interfingering landwards with siliciclastic deposits of a mixed siliciclastic-carbonate shelf (Fito Fm.) adjacent to deltaic systems. The spatial variability of the carbonate facies and the high-frequency (4th–5th order) cycles, from the platform margin-outer platform to the deltaic systems, has been interpreted from basin reconstruction. Carbonate facies include skeletal grainstone to packstone, ooidal grainstone, burrowed skeletal wackestone, microbial and algal boundstone to wackestone forming mounds, various algal bafflestone and coral biostromes in areas with siliciclastic input. These high-frequency transgressive–regressive cycles are interpreted to record allocyclic forcing of high-amplitude glacioeustasy because they show characteristic features of icehouse cycles: thickness >5 m, absence of peritidal facies, and in some cases, subaerial exposure surfaces capping the cycles. In the mixed cycles, siliciclastics are interpreted as late highstand to lowstand regressive deposits, whereas carbonates as transgressive-early highstand deposition. The lateral and vertical variability of the facies in the glacioeustatic cycles was a response to deposition in a rapidly subsiding, active foreland basin subjected to siliciclastic input, conditions that might be detrimental to the growth of high-relief carbonate systems.

Ecomorphological and taphonomic gradient of clypeasteroid-dominated echinoid assemblages along a mixed siliciclastic-carbonate shelf from the early Miocene of northern Sardinia, Italy

Ecomorphological and taphonomic gradient of clypeasteroid-dominated echinoid assemblages along a mixed siliciclastic-carbonate shelf from the early Miocene of northern Sardinia, Italy

An overview of the Upper Palaeozoic–Mesozoic stratigraphy of the NE Atlantic region

Abstract This study describes the distribution and stratigraphic range of the Upper Palaeozoic–Mesozoic succession in the NE Atlantic region, and is correlated between conjugate margins and along the axis of the NE Atlantic rift system. The stratigraphic framework has yielded important new constraints on the timing and nature of sedimentary basin development in the NE Atlantic, with implications for rifting and the break-up of the Pangaean supercontinent. From a regional perspective, the Permian–Triassic succession records a northwards transition from an arid interior to a passively subsiding, mixed carbonate–siliciclastic shelf margin. A Late Permian–earliest Triassic rift pulse has regional expression in the stratigraphic record. A fragmentary paralic to shallow-marine Lower Jurassic succession reflects Early Jurassic thermal subsidence and mild extensional tectonism; this was interrupted by widespread Mid-Jurassic uplift and erosion, and followed by an intense phase of Late Jurassic rifting in some (but not all) parts of the NE Atlantic region. The Cretaceous succession is dominated by thick basinal-marine deposits, which accumulated within and along a broad zone of extension and subsidence between Rockall and NE Greenland. There is no evidence for a substantive and continuous rift system along the proto-NE Atlantic until the Late Cretaceous.

The Vaca Muerta–Quintuco system (Tithonian–Valanginian) in the Neuquén Basin, Argentina: A view from the outcrops in the Chos Malal fold and thrust belt

The Vaca Muerta–Quintuco system (uppermost lower Tithonian–lower Valanginian) is a thick shallowing-upward sedimentary cycle consisting of dark bituminous shales, marlstones, limestones, and sandstones, cropping out in the Neuquen Basin, west–central Argentina. This paper analyzes three outcrop sections in Chos Malal area, northem Neuquen province. Detailed facies analysis allows us to differentiate six facies associations, representing basinal to proximal outer ramp facies of a homoclinal carbonate ramp system (Vaca Muerta Formation) and basinal to shoreface facies of a mixed carbonate–siliciclastic shelf system (Quintuco Formation), prograding westward from the eastern margin of the basin. Two sequence hierarchies were recognized: 5 composite depositional sequences (third order) and 15 high-frequency sequences (fourth order). Fluctuations in organic matter content within the Vaca Muerta Formation suggest a relationship with depositional sequences, finding the highest values associated with transgressive systems tract, whereas the transition to the Quintuco Formation shows a strong decrease in total organic carbon. The x-ray diffraction studies show an increase of clay minerals and quartz in the transgressive systems tract of the Vaca Muerta Formation and an increase in the content of calcite in highstand systems tracts. This pattern is reversed in the Quintuco Formation. Our sequence stratigraphic approach contributes to the understanding of the relationship between organic matter, clay minerals, facies, stacking pattern, and relative sea level changes in this exceptional shale oil and shale gas unconventional reservoir. This study may be helpful for a better postulate of petrophysical and geomechanical models for unconventional exploration.

Shelf morphology as an indicator of sedimentary regimes: A synthesis from a mixed siliciclastic–carbonate shelf on the eastern Brazilian margin

Modern shelf morphology is the result of the interplay between short and long term sedimentary processes. The relation between rates of sediment supply/carbonate growth and accommodation space creation will not only control coastal transgression and regression, but will also define the shelf sedimentary regimes acting to shape the seabed. Herein, shelf morphology and sedimentology are investigated in order to discuss how these characteristics can be representative of distinct sedimentary regimes. The study area is the eastern Brazilian shelf where coastal transgression and regression coexist with the most important coral reef system of the South Atlantic. A compilation of existing published and unpublished data was carried out in order to produce morphological and faciological maps and compare the mapped features with high-resolution seismic and sonographic data. The results show three major regions or morphological compartments: Abrolhos Shelf, Doce River Shelf and the Paleovalleys Shelf. In terms of shelf sedimentary domain, rhodolith beds predominate over the outer shelf along the entire area, coralline reefs are present along the northern Abrolhos inner shelf and a significant terrigenous mud deposit is observed associated to the Doce River adjacent inner shelf beds. The rest of the shelf is composed by bioclastic or terrigenous mud sand and gravel. Terrigenous sedimentation is always restricted to the shoreface or inner shelf shallower areas and carbonate sands and gravels are predominant elsewhere. The Abrolhos shelf shows two distinct sectors; the northern area is a typical mixed sediment environment that has a supply regime along the coast/shoreface, mainly due to longshore transport and a carbonate regime along the inner and outer shelf. The southern shelf morphology and sedimentation are controlled by the antecedent topography and is typically a accommodation regime shelf with associated rhodolith beds. The Doce river shelf is a supply regime environment with the formation of a 5–8 m thick regressive deposit with downlapping clinoforms. Southward from the Doce river shelf, a significant shift in sedimentary regime is observed as the morphology becomes very irregular with associated hardbottoms and unfilled paleovalleys. This sector of the shelf (Paleovalley shelf) is characterized by an accommodation regime. The interpretation shows that the entire study area can be defined as a mixed sedimentation shelf, showing supply and accommodation regimes. Shelf morphology worked as an indicator of these changes. Carbonate/terrigenous deposition during a highstand/regressive phase coeval along the eastern Brazilian shelf, either laterally and across shelf. This lateral/along coast variation in sediment supply and carbonate production leads to distinct lateral facies and geometry. These spatial changes in morphology and facies, with coexistence of carbonate and siliciclastic sedimentation, are very important for the correlation and interpretation of the geological record, especially stratigraphic surfaces and sequence units.

Sedimentology, chemostratigraphy, and stromatolites of lower Paleoproterozoic carbonates, Turee Creek Group, Western Australia

The ca. 2.45–2.22Ga Turee Creek Group, Western Australia, contains carbonate-rich horizons that postdate earliest Proterozoic iron formations, bracket both Paleoproterozoic glaciogenic beds and the onset of the Great Oxidation Event (GOE), and predate ca. 2.2–2.05Ga Lomagundi-Jatuli C-isotopic excursion(s). As such, Turee Creek carbonate strata provide an opportunity to characterize early Paleoproterozoic carbonate sedimentation and carbon cycle dynamics in the context of significant global change. Here, we report on the stratigraphy, sedimentology, petrology, carbon isotope chemostratigraphy, and stromatolite development for carbonate-rich successions within the pre-glacial part of the Kungarra Formation and the postglacial Kazput Formation.Kungarra carbonate units largely occur as laterally discontinuous beds within a thick, predominantly siliciclastic shelf deposit. While this succession contains thin microbialite horizons, most carbonates consist of patchy calcite overgrowths within a siliciclastic matrix. C-isotopic values show marked variation along a single horizon and even within hand samples, reflecting spatially and temporally variable mixing between dissolved inorganic carbon in seawater and isotopically light inorganic carbon generated via syn- and post-depositional remineralization of organic matter.In contrast, the Kazput carbonates consist of subtidal stromatolites, grainstones, and micrites deposited on a mixed carbonate–siliciclastic shelf. These carbonates exhibit moderate δ13C values of −2‰ to +1.5‰ and likely preserve a C-isotopic signature of seawater. Kazput carbonates, thus, provide some of the best available evidence that an interval of unexceptional C-isotopic values separates the Lomagundi-Jatuli C-isotopic excursion(s) from the initiation of the GOE as inferred from multiple sulfur isotopes (loss of mass independent fractionation). The Kazput Formation also contains unusual, m-scale stromatolitic buildups, which are composed of sub-mm laminae and discontinuous, convex upward lenticular precipitates up to a few mm in maximum thickness. Laminae, interpreted as microbial mat layers, contain quartz and clay minerals as well as calcite, whereas precipitate lenses consist of interlocking calcite anhedra, sometimes showing faint mm-scale banding. These cements formed either as infillings of primary voids formed by gas emission within penecontemporaneously lithified mats, or as local seafloor precipitates that formed on, or within, surface mats. It is possible that both mechanisms interacted to form the unique Kazput stromatolites. These microbialites speak to a distinctive interaction between life and environment early in the Paleoproterozoic Era.

The Modern Mixed Carbonate-Siliciclastic Abrolhos Shelf: Implications For A Mixed Depositional Model

Abstract High-resolution seismic facies analysis and stratigraphy of a mixed carbonate–siliciclastic shelf yields insights into sedimentation controlled by interactions among carbonate production, siliciclastic sediment supply, and changes in relative sea level. High-resolution seismic, core, and surface samples collected along the northern area of the Abrolhos shelf (Brazil), including the inner, middle, and outer shelf, constrain the genesis of the shelf. Two sets of seismic sequences were identified: a Pleistocene sequence and Pleistocene–Holocene sequence. A subaerial unconformity from subaerial exposure during the last glacial maximum separates the two sequences. The upper seismic sequence, the focus of this study, comprises retrograding–aggrading (transgressive) and aggrading–prograding (regressive) seismic geometries. Carbonate sedimentation predominated within the last postglacial transgressive phase. The subsequent regressive phase was dominated by nearshore siliciclastic sedimentation at shallow depths (up to ∼ 12 m). Below 12 m depth, a transition occurs to mixed siliciclastic–carbonate facies, and ultimately, carbonate sedimentation farther offshore. Seismic data indicate that Holocene nearshore coral reef growth was influenced by the positive antecedent relief of a Pleistocene reef platform and terrigenous sedimentation. Although siliciclastic–carbonate mixed sedimentation has been reported in other successions during periods of regression, siliciclastic sedimentation in northern Abrolhos shelf is limited to nearshore settings rather than bypassing the shelf.

The investigation of a mixed carbonate-siliciclastic shelf, NE Brazil: side-scan sonar imagery, underwater photography, and surface-sediment data

Side-scan sonar imagery and underwater photography, coupled with bathymetric and sedimentologic data, revealed noteworthy features and the interplay of multiple processes on the north-eastern Brazilian continental shelf. The high-resolution data cover a 500 km2 area of the shallow (~70 m) and narrow (~40 km) shelf, which comprises the submerged portion of the Potiguar Basin; the area includes the partially filled incised valley of the ancient Acu River.The seafloor imagery was integrated with 673 surface sediment samples to distinguish boundaries within the mixed siliciclastic-carbonate region. The sediment grain size increases from nearshore, muddy, siliciclastic sediments to biogenic, coarse sand and gravel on the outer shelf. Carbonate-rich sediment predominates, except on the eastern margin of the Acu Incised Valley (AIV), the nearshore zone and estuary mouths. The AIV is filled mainly with muddy sediments reworked from the latest transgressive deposits. Bedform configurations indicate bottom currents associated with tidal and wave-driven currents.Mixing processes are controlled by the interplay of high hydrodynamic energy regime and shelf physiography. Special attention was paid to the middle and outer shelves, where a large field of newly found reefs occurs between 20 and 50 m depths. The reef substrate is composed of limestones, probably belonging to the Guamare Formation.SCUBA dive inspections revealed benthic colonies (corals: Montastraea cavernosa, Porites astreoides, Siderastrea stellata; sponges: Scopalina ruetzleri, Callyspongia vaginalis, Ectyoplasia ferox, Ircinia sp., Spirastrella sp., Aiolochroia sp.) over fossil reefs. A preliminary examination of the foraminiferal species showed a Caribbean-type reef community, including Amphisorus hemprichii, Amphistegina gibbosa, Archaias angulatus, Heterostegina antillarum, Homotrema rubra, Peneroplis carinatus and P. proteus.

Temperate carbonate debrites and short-lived earliest Miocene yo-yo tectonics, eastern Taranaki Basin margin, New Zealand

This study examines two unconformity-bound earliest Miocene temperate shallow-marine limestones at remote Gibson Beach, western North Island, on the eastern margin of Taranaki Basin, New Zealand's only producing hydrocarbon province. The local geology comprises an upper limestone of up to 6m of spectacular conglomeratic limestone (rudstone; Papakura Limestone) whose pebble- to boulder-sized clasts were derived from cannibalisation of the lower skeletal limestone (bryomol grainstone; Otorohanga Limestone). Multiple lines of evidence require the Otorohanga Limestone to have been well indurated before erosion and clast generation. Cementation occurred from pressure dissolution of calcitic skeletons at burial depths of probably 400m or more. Subsequent uplift led to local subaerial exposure and karstification of a fault-block cliffed coastline where physical erosion sourced talus limestone debris onto adjacent high-energy pocket beaches. Storm and/or seismic events periodically triggered mass flows of the limestone clasts offshore onto the contemporary mixed siliciclastic-carbonate shelf as channelised carbonate debrites. Lithification of these Papakura Limestone debrites occurred via pressure dissolution, and required a second burial episode. Karst pinnacles preserved on the eroded upper surface of the Papakura Limestone demand uplift again and erosion in a subaerial setting, before subsequent deep burial by Early Miocene siliciclastic shelf sand and turbidite deposits. Strontium isotope dating of brachiopod fossils suggests the two postulated burial-uplift cycles, involving movements up to 400±100m, occurred very rapidly within several 100kyr. Speculative drivers of the yo-yo tectonic events are earliest Miocene movements on the nearby major Taranaki Fault thrust in association with regional changes in subduction tectonics at the Australian-Pacific plate boundary, along with pulsed emplacement of obducted gravity slide deposits (Northland Allochthon) onto northern North Island. The two limestones record a previously undocumented degree of eastern Taranaki Basin margin tectonic mobility relevant to a fuller appreciation of petroleum system development in that basin.

Quantitative comparisons and models of time-averaging in bivalve and brachiopod shell accumulations

The variation in time-averaging between different types of marine skeletal accumulations within a depositional system is not well understood. Here we provide quantitative data on the magnitude of time-averaging and the age structure of the sub-fossil record of two species with divergent physical and ecological characteristics, the brachiopodBouchardia roseaand the bivalveSemele casali.Material was collected from two sites on a mixed carbonate-siliciclastic shelf off the coast of Brazil where both species are dominant components of the local fauna.Individual shells (n= 178) were dated using amino acid racemization (aspartic acid) calibrated with 24 AMS radiocarbon dates. Shell ages range from modern to 8118 yearsb.p.for brachiopods, and modern to 4437 years for bivalves. Significant differences in the shape and central tendency of age-frequency distributions are apparent between each sample. Such differences in time-averaging magnitude confirm the assumption that taphonomic processes are subject to stochastic variation at all spatial and temporal scales. Despite these differences, each sample is temporally incomplete at centennial resolution and three of the four samples have similar right-skewed age-frequency distributions. Simulations of temporal completeness indicate that samples of both species from the shallow site are consistent with a more strongly right-skewed and less-complete age-frequency distribution than those from the deep site.We conclude that intrinsic characteristics of each species exert less control on the time-averaging signature of these samples than do extrinsic factors such as variation in rates of sedimentation and taphonomic destruction. This suggests that brachiopod-dominated and bivalve-dominated shell accumulations may be more similar in temporal resolution than previously thought, and that the temporal resolution of multi-taxic shell accumulations may depend more on site-to-site differences than on the intrinsic properties of the constituent organisms.

High-Frequency Rhythmicity in a Mixed Siliciclastic-Carbonate Shelf (Late Miocene, Guadix Basin, Spain): A Model of Interplay Between Climatic Oscillations, Subsidence, and Sediment Dispersal

Abstract Rhythmites in pelagic sediments can include information on paleoclimate and with orbital cycles. This paper documents and interprets an unusual rhythmic succession from a cool-water shallow-marine shelf that includes features of Mediterranean-type ramps and open-ocean-type ramps. In the subsiding southwestern margin of the Guadix Basin (southern Spain, Betic Cordillera), during the late Tortonian, a 640-m-thick succession was deposited over a short period (7.8–7.2 Ma). This succession contains 30 sedimentary packages or rhythmites, each defined by two facies associations: a mixed cross-stratified siliciclastic–skeletal sandstone and a homogeneous marl rich in planktonic foraminifera. Sandstones ranging in thickness from 3 m to 20 m contain coralline algae, bivalves, bryozoans, benthic foraminifera, echinoids, and brachiopods (typical temperate-water shelf biofacies). These units exhibit cross-strata sets up to four meters high, interpreted to have been produced by strong unidirectional currents. The marly deposits range in thickness from 2 to 17 m and contain an association of cold-water planktonic foraminifera as well as thin sandstone layers that exhibit normal grading corresponding to typical incomplete Bouma sequences or a lower division with planar lamination and an upper division of very fine- to fine-grained sandstone with hummocky and swaly cross-stratification. On the basis of the sedimentary structures, the sandstone layers are interpreted as sporadic turbidity flows and storm ebb–surge deposits. On the basis of the sedimentary structures, the hydrodynamic conditions during deposition of the mixed siliciclastic–carbonate interval were high-energy. Based on sedimentary and biogenic features of the rhythmites and their temporal durations (ca. 20 ka), rhythmicity is interpreted to reflect alternating cool-wet and cold-dry climate episodes driven by precession orbital cycles. This occurrence of alternating mixed carbonate–siliciclastic sedimentation and pelagic sediments represents a unique occurrence in the geologic record.

PALYNOMORPH DISTRIBUTION IN MODERN TROPICAL DELTAIC AND SHELF SEDIMENTS - MAHAKAM DELTA, BORNEO, INDONESIA

Abstract The Mahakam Delta is a tropical, fluvio—tidally dominated delta prograding onto a mixed siliciclastic—carbonate shelf. Surface sediments were collected from 12 distinctive depositional environments extending from the head of the Mahakam Delta to the shelf edge. Organic matter was extracted from 200 samples by acid digestion, and seven palynomorph types (pollen, embryophyte spores, fungal spores, foraminiferal linings, copepod eggs, tintinnomorphs, and dinocysts) were identified and counted, and their percentages and concentrations calculated. Total organic carbon (TOC), lithology, and depositional environments were recorded for each sediment sample. The amounts of terrestrial palynomorphs in sediments can be explained by transport and depositional processes, and four groups of environments are identified (tidally influenced, detrital peat beach, lowland rain forest, and marine shelf). Because of tidal flushing of the delta plain, pollen, embryophyte spores, fungal spores and soft plant debris fro...

Mixed siliciclastic-carbonate shelf sedimentation—Lower Devonian sequences of the SW Anti-Atlas, Morocco

In the south-western Anti-Atlas of Morocco (Dra Plain), a continuous exposure of Lower Devonian sedimentary successions more than 400 km long provides an example of mixed siliciclastic-carbonate shelf sequences. The carbonates were deposited adjacent and down-dip to large lobes of storm-dominated deltaic complexes during sea-level lowstands, and subsequently transgressed over the 200 m thick prograding siliciclastic wedges during sea-level rise, depositing limestone units between 5 and 20 m thick. The deltaic lobes switched through time and caused alternating siliciclastic supply along the shoreline oblique–parallel transect. Condensed limestone successions formed in clastic sediment starved areas due to delta-complex abandonment. Down-gradient, in outer ramp environments, the direct juxtaposition of carbonate and siliciclastic lithofacies associations comprising rhythmic lime mudstones/marls and massive shelf sandstones, provides the opportunity for mutual interpretation of their mode of deposition and bathymetry. This lateral relationship suggests that significant amalgamation took place in falling stage systems tracts to produce laterally extensive shelf sandstone-sheets on the outer ramp. Two types of stacking patterns are observed in prograding siliciclastic wedges. A normal progradational shoreline pattern with well developed parasequence sets in the proximal ramp, and an abrupt-regressive succession in which the amalgamated shelf sandstones rest more or less directly on offshore siltstones in the distal ramp. In the latter case, the falling stage systems tract sandstones are overlain by a relatively thick cyclic alternation with brachiopod-rich storm-beds below a marked transgressive surface at the base of the capping transgressive limestones. The cyclic alternation is interpreted as lowstand systems tract deposits. Marked flooding surfaces on parasequence sets are Fe-enriched and contain abundant articulated brachiopods, indicating breaks in sedimentation and an overall deepening-upward facies from the underlying maximum regression and sequence boundary. These aggradational to retrogradational stacked lowstand systems tracts are thus rather ‘transgressive’, not regressive in character, as is commonly described in sequence stratigraphic ramp models.