Gearle Siltstone Research Articles

Anacoracid sharks and calcareous nannofossil stratigraphy of the mid-Cretaceous ‘upper’ Gearle Siltstone and Haycock Marl in the lower Murchison River area, Western Australia

Siversson, M., Cook, T.D., Ryan, H.E., Watkins, D.K., Tatarnic, N.J., Downes, P.J. & Newbrey, M.G. 5 June 2018. Anacoracid sharks and calcareous nannofossil stratigraphy of the mid-Cretaceous Gearle Siltstone and Haycock Marl in the lower Murchison River area, Western Australia. Alcheringa 43, 85–113.Extensive bulk sampling over the past 20 years and greatly improved stratigraphic control permitted a meaningful revision of previously described anacoracid sharks from the ‘upper’ Gearle Siltstone and lower Haycock Marl in the lower Murchison River area, Western Australia. Isolated teeth of anacoracids are rare in the lower three (Beds 1–3) of four stratigraphic units of the ‘upper’ Gearle Siltstone but relatively common in the uppermost layer (Bed 4) and in the lower part of the overlying Haycock Marl. On the basis of calcareous nannofossils, Beds 1 and 2 of the ‘upper’ Gearle Siltstone can be placed in the uppermost upper Albian calcareous nannofossil Subzone CC9b whereas Bed 3 can be referred to the lowermost Cenomanian CC9c Subzone. Bed 1 yielded fragments of strongly serrated anacoracid teeth as well as a single, smooth-edged tooth. The samples from Beds 2 and 3 contained a few small fragments of serrated anacoracid teeth. Bed 4 is barren of calcareous nannofossils but the presence of a dentally advanced tooth of the cosmopolitan lamniform genus Cretoxyrhina in combination with the age of the overlying Haycock Marl indicate deposition within the younger half of the Cenomanian. The unit produced teeth of two anacoracids; Squalicorax acutus sp. nov. and S. bazzii sp. nov. The basal, laminated part of the Haycock Marl is placed in the uppermost upper Cenomanian part of CC10b. It yielded Squalicorax mutabilis sp. nov. and S. aff. S. bernardezi. Exceptionally well-preserved teeth of the former species span a 5:1 size ratio range for teeth of comparable jaw position. The teeth reveal strong ontogenetic heterodonty with a large increase in the relative size of the main cusp with age and the transition from a vertical distal heel of the crown in very young juveniles to a sub-horizontal, well demarcated heel in ‘adult’ teeth. An isolated phosphatic lens in the lower part of the Haycock Marl produced calcareous nannofossils indicative of the CC10b SubZone, most likely the lowermost lower Turonian part. It contains teeth of Squalicorax mutabilis sp. nov., S. aff. S. bernardezi, and S. sp. C.Mikael Siversson* [mikael.siversson@museum.wa.gov.au], Helen E. Ryan [helen.ryan@museum.wa.gov.au] and Peter Downes [peter.downes@museum.wa.gov.au] Department of Earth and Planetary Sciences, Western Australian Museum, 49 Kew Street, Welshpool, Western Australia 6106, Australia; David K. Watkins [dwatkins@unl.edu] Department of Earth and Atmospheric Sciences, University of Nebraska, Lincoln, NE 68588, USA; Todd D. Cook [tdc15@psu.edu] School of Science, Penn State Behrend, 4205 College Drive, Erie, PA 16563, USA; Nikolai J. Tatarnic† [nikolai.tatarnic@museum.wa.gov.au] Department of Terrestrial Zoology, Western Australian Museum, 49 Kew Street, Welshpool, Western Australia 6106, Australia; Michael G. Newbrey‡ [newbrey_michael@columbusstate.edu] Department of Biology, Columbus State University, Columbus, GA 31907-5645, USA. *Also affiliated with: School of Molecular and Life Sciences, Curtin University, Kent Street, Bentley, WA 6102, Australia. †Also affiliated with: Centre for Evolutionary Biology, University of Western Australia, Crawley, Western Australia 6009. ‡Also affiliated with: Canadian Fossil Discovery Centre, 111-B Gilmour Street, Morden, Manitoba R6 M 1N9, Canada.http://zoobank.org/urn:lsid:zoobank.org:pub:97D5131F-C0D5-4A7E-9C9A-0FDF13BFCBBBhttp://zoobank.org/urn:lsid:zoobank.org:act:5977DCC2-355C-4732-8B0A-4BD0EABBA8DEhttp://zoobank.org/urn:lsid:zoobank.org:act:2D7C4147-B756-4434-847A-B0C1C6D167DFhttp://zoobank.org/urn:lsid:zoobank.org:act:33F3B55E-41E0-45B3-8296-A3B95C17B41D

Pumiliobelus, a new dwarf coleoid genus (Belemnoidea: Dimitobelidae) from the Cenomanian of Western Australia

AbstractA rare diminutive belemnite, Pumiliobelus n. gen., is described based on the new species P. haigi and P. tumidus from the upper Gearle Siltstone (Cenomanian) of the Carnarvon Basin, Western Australia. No smaller belemnites than these species are known. That they are based on adult specimens rather than juveniles is supported by a large sample, some 80 specimens, of P. haigi which collectively show a proportional range in size typical of belemnite species in general, a group in which juveniles are conspicuously lacking in the fossil record. Rostral morphology places Pumiliobelus in the Dimitobelidae, a distinctively Austral family of Aptian–Maastrichtian age, which became progressively restricted to high latitudes through the late Cretaceous. Each species of Pumiliobelus is known from a single locality where it co-occurs with the long-ranging (Albian–Cenomanian) Dimitobelus diptychus Whitehouse of widespread distribution at mid to high latitudes in the Southern Hemisphere. Dwarfism in the Dimitobelidae, as expressed by Pumiliobelus, is considered to reflect adaptive evolution to engage opportunistic life strategies that favored small body size and rapid population turnover. The rare occurrence of dwarf Dimitobelidae indicates that such strategies were limited in both geographic range and duration. Dwarfism expressed by Pumiliobelus may relate to paedomorphosis induced by rising seawater temperatures in the mid Cretaceous.

Late Cretaceous (Cenomanian-Campanian) mid-palaeolatitude sharks of Cretalamna appendiculata type

The type species of the extinct lamniform genus Cretalamna, C. appendiculata, has been assigned a 50 Ma range (Albian—Ypresian) by a majority of previous authors. Analysis of a partly articulated dentition of a Cretalamna from the Smoky Hill Chalk, Kansas, USA (LACM 128126) and isolated teeth of the genus from Cenomanian to Campanian strata of Western Australia, France, Sweden, and the Western Interior of North America, indicates that the name of the type species, as applied to fossil material over the last 50 years, represents a large species complex. The middle Cenomanian part of the Gearle Siltstone, Western Australia, yielded C. catoxodon sp. nov. and “Cretalamna” gunsoni. The latter, reassigned to the new genus Kenolamna, shares several dental features with the Paleocene Palaeocarcharodon. Early Turonian strata in France produced the type species C. appendiculata, C. deschutteri sp. nov., and C. gertericorum sp. nov. Cretalamna teeth from the late Coniacian part of the Smoky Hill Chalk in Kansas are ...

Albian (Lower Cretaceous) Dinoflagellate cyst biostratigraphy of the Lower Gearle Siltstone, Southern Carnarvon Basin, Western Australia

Rich and diverse assemblages of dinoflagellate cysts were recovered from the Gearle Siltstone in the Boologooro‐1 stratigraphic well, Southern Carnarvon Basin, Western Australia. The cored interval through the Winning Group is proposed as a reference interval for the Canninginopsis denticulata, Endoceratium ludbrookiae, Dioxya armata and Xenascus asperatus dinoflagellate cyst zones of the Australian dinoflagellate cyst zonal scheme. The ranges of sixty‐three dinoflagellate cyst species are documented in detail over this interval. Apart from the bioevents that define the widely‐used dinoflagellate cyst subzones, the highest occurrences of Endoceratium turneri, Craspedodinium indistinctum and Litosphaeridium arundum, and the lowest occurrences of Nematosphaeropsis densiradiata,Canninginopsis denticulata, Leberidocysta chlamydata, Aptea? sp. cf. Aptea polymorpha, Litosphaeridium siphoniphorum, Balcattia cirribarbata and Stephodinium australicum are recognized as potential biostratigraphic datums in the Southern Carnarvon Basin. Species diversity increased in the highest Canninginopsis denticulata Zone at approximately the horizon where Diconodinium spp. become dominant. Foraminiferal data indicate the interval from 361.15 m to 312.12 m is marginally inner neritic and corresponds with an interval of abundant Diconodinium spp., and fewer Spiniferites spp. Diversity increased in the Xenascus asperatus Zone with an increase in Spiniferites spp., and a shift to middle neritic paleobathymetry. The age of the top of the Xenascus asperatus Zone is constrained by calcareous nannofossils and dinoflagellate cyst correlations to the latest Albian, or possibly the early Cenomanian. The age of the base of the Canninginopsis denticulata Zone is constrained by dinoflagellate cyst and foraminiferal data to the early Albian, above the earliest Albian. One new species, Chlamydophorella haigii, is erected and aspects of several other dinoflagellate cyst taxa are reviewed.

ALBIAN (LOWER CRETACEOUS) DINOFLAGELLATE CYST BIOSTRATIGRAPHY OF THE LOWER GEARLE SILTSTONE, SOUTHERN CARNARVON BASIN, WESTERN AUSTRALIA

Abstract Rich and diverse assemblages of dinoflagellate cysts were recovered from the Gearle Siltstone in the Boologooro-1 stratigraphic well, Southern Carnarvon Basin, Western Australia. The cored interval through the Winning Group is proposed as a reference interval for the Canninginopsis denticulata, Endoceratium ludbrookiae, Dioxya armata and Xenascus asperatus dinoflagellate cyst zones of the Australian dinoflagellate cyst zonal scheme. The ranges of sixty-three dinoflagellate cyst species are documented in detail over this interval. Apart from the bioevents that define the widely-used dinoflagellate cyst subzones, the highest occurrences of Endoceratium turneri, Craspedodinium indistinctum and Litosphaeridium arundum, and the lowest occurrences of Nematosphaeropsis densiradiata, Canninginopsis denticulata, Leberidocysta chlamydata, Aptea? sp. cf. Aptea polymorpha, Litosphaeridium siphoniphorum, Balcattia cirribarbata and Stephodinium australicum are recognized as potential biostratigraphic datums in...

DETECTING OVERPRESSURE USING POROSITY-BASED TECHNIQUES IN THE CARNARVON BASIN, AUSTRALIA

Overpressure has been encountered in many wells drilled in the Carnarvon Basin. Sonic logs are used to estimate pore pressure in shales in the Carnarvon Basin using the Eaton and equivalent depth methods of estimating pore pressure from velocity data with reference to a normal compaction trend. The crux of pore pressure estimation from the sonic log lies in the determination of the normal compaction trend, i.e. the acoustic travel time (Δt)/depth (z) trend for normally pressured sediments. The normal compaction trend for shales in the Carnarvon Basin was established by fitting an Athy-type exponential relationship to edited sonic log data, and is: Δt = 225 + 391exp(-0.00103z) Vertical stress estimates are also needed for the Eaton and equivalent depth methods used herein. A vertical stress (σv) relationship was obtained by fitting a regression line to vertical stress estimates from the density log, and is: σv = 0.0131 z1.0642 The Eaton and equivalent depth methods yield similar pressure estimates. However, the equivalent depth method can only be applied over a limited range of acoustic travel times, a limitation that does not apply to the Eaton method. The pressure estimates from the Eaton method were compared to pressures measured by direct pressure tests in adjacent permeable units. There is a good correlation between Eaton and test pressures in normally pressured intervals in three wells and overpressured intervals in two wells. Eaton pressure estimates underestimate overpressured direct pressure measurements in four wells by up to 13 MPa. This is consistent with overpressure being generated (at least in part) by a fluid expansion mechanism or lateral transfer of overpressure. The Eaton pressures in one well are, on average, 11 MPa lower than hydrostatic pore pressure recorded in direct pressure measurements below the Muderong Shale. The sediments in this well appear to be overcompacted due to exhumation. Mud weights can be used as a proxy for pore pressure in shales where direct pressure measurements are not available in the adjacent sandstones. The Eaton pressure estimates are consistent with mud weight in the Gearle Siltstone and Muderong Shale in 4 of the 8 wells studied. The Eaton pressures are on average 10 Mpa in excess of mud weight in the Muderong Shale and Gearle Siltstone in three wells. It is unclear whether the predicted Eaton pressures in these three wells accurately reflect pore pressure (i.e. the mud weights do not accurately reflect pore pressure), or whether they are influenced by changes in shale mineralogy (because the gamma ray filter does not differentiate between shale mineralogy).

Cenomanian–Coniacian transition from siliciclastic to carbonate marine deposition, Giralia Anticline, Southern Carnarvon Platform, Western Australia

Three formations exposed in the Giralia Anticline record the transition from siliciclastic to carbonate-dominated sedimentation on the Southern Carnarvon Platform, a stable shelf on the passive western Australian continental margin. The succession consists of condensed and partly eroded sequences that record maximum marine flooding events and the unconformities between them. The stratigraphic record of the depositional change is evident in four <2-m-thick sections analysed here by outcrop logging and micropalaeontological analysis. The oldest sequence, at 96–94Ma, includes transgressive-phase deposits truncated by an unconformity, but reflecting a maximum water depth of less than 50m in the preserved record. This sequence is represented in the uppermost 0.5m of the Gearle Siltstone in its type area as a dark grey mudstone that belongs to nannofossil Zone CC10a (Middle Cenomanian). The second sequence, dated at 92–91Ma, is evidenced by a very thin (<0.25m thick) unit of yellowish brown to pale yellow marl that belongs to the upper Lower Turonian (based on the overlap of the upper part of nannofossil Zone CC11 and the planktic foraminiferal Helvetoglobotruncana helvetica Zone). A middle neritic (water depth about 100m) depositional setting is suggested by the foraminiferal assemblage. The transgressive base of the third sequence (dated at 88–87Ma) includes the lowest 20cm of the Toolonga Calcilutite that contains abundant nannofossils indicative of zone CC14 (mid Coniacian). The basal marls and calcilutites of this sequence were deposited on a marine erosion surface at water depths of about 50m. There is broad correspondence between the local record of bathymetric change and the eustatic sea-level curve for the mid Cenomanian to mid Coniacian interval. In shallow waters (50–100m deep) relatively close to the shoreline, influx of siliciclastic mud decreased allowing accumulation of nannofossiliferous muds during the Turonian and Coniacian interval. A decrease in the amount of precipitation, erosion and river transport of sediment onto the continental shelf (indicative of climatic change, perhaps to more arid conditions in the hinterland) was probably the main factor that led to the change from marine siliciclastic to carbonate sedimentation that occurred between 96–94Ma and 92–91Ma in the Giralia region.

Quantification of uplift in the Carnarvon Basin based on interval velocities

Sonic velocities in the Toolonga Calcilutite (Upper Cretaceous), the Gearle Siltstone and Haycock Marl (mid‐Cretaceous), and the Muderong Shale (Lower Cretaceous) were each used to independently quantify apparent uplift (height above maximum burial depth) in the Carnarvon Basin. Apparent uplift is given by the difference in depth between the present‐day velocity‐depth trend for a unit in a particular well and the reference trend (unaffected by uplift) of the unit. Apparent uplift results derived from the Toolonga Calcilutite, Gearle Siltstone and Muderong Shale are statistically similar. The consistency of results from carbonate and clastic units suggests that, at the formation and regional scale, overcompaction (i.e. anomalously high interval velocity) in these three units reflects previously greater burial depth, rather than sedimentological and/or diagenetic processes, and validates the use of lithologies other than shale in maximum burial‐depth studies. The proposed magnitudes of apparent uplift are greatest along some of the main structural highs of the Carnarvon Basin. Apparent uplift of approximately 800 m was determined along the Legendre Trend, and along the Barrow Arch. In excess of 900 m of apparent uplift was determined on the Bambra Anticline. The consistency of results from units of Early to Late Cretaceous age suggests that uplift must have post‐dated the youngest (Late Cretaceous) unit analysed.

A new large lamniform shark from the uppermost Gearle Siltstone (Cenomanian, Late Cretaceous) of Western Australia

AbstractAn association of 100 teeth and 15 vertebrae from a large, lamniform shark, is described from the uppermost part of the Gearle Siltstone in the Giralia Anticline, Southern Carnarvon Basin, Western Australia. The material represents a new genus and species: Cardabiodon ricki. The taxon is referred to the new family Cardabiodontidae on the basis of its dental formula and uniquely oversized lower lateroposterior teeth. Other characteristic features include a strong dignathic heterodonty and the presence of four anterior and approximately 14 lateroposterior toothfiles in both the upper and lower jaw. The size of the recovered vertebral centra indicates that the shark measured at least 5 m in total length. The species had a wide but possibly patchy distribution with additional occurrences in England and Kazakstan, where it is present in strata of Cenomanian age. Cardabiodon shares several key dental characters with Parotodus, known from Oligocene–Pliocene deposits. The latter taxon was previously grouped with the ‘mega-toothed’ sharks of the genera Otodus and Carcharocles, but is here reassigned to the Cardabiodontidae.The nominal species Cretalamna woodwardi from the middle or late Cenomanian of southern England, is designated as the type species of the new genus Dwardius. The coeval nominal species Pseudoisurus tomosus from the Saratov region in Russia, is based on material that almost certainly includes C. ricki and/or D. woodwardi. Designation of a lectotype for P. tomosus and its proper documentation would probably allow either C. ricki or D. woodwardi to be synonomised with the older name P. tomosus. There is, however, a strong possibility that the syntypes of P. tomosus are lost. Until they are found, redescribed and compared carefully with the type material of C. ricki and D. woodwardi, P. tomosus is a name of doubtful application and therefore referred to as a women dubium

BARROW ISLAND OIL FIELD

The Barrow Island Field in the Barrow Sub-basin of the Carnarvon Basin was discovered in 1964 by West Australian Petroleum Pty Limited. It is the largest oil field in Western Australia. Appraisal drilling has defined in-place oil of 200 GL (1,250 MMBBL) and in-place gas of 16.5 Gm3 (580 BCF) primarily in the Lower Cretaceous Windalia Sand Member of the Muderong Shale and in- place gas of 14.5 Gm3 (515 BCF) in Middle Jurassic Biggada Formation. Additional hydrocarbon reservoirs have been discovered, including oil and gas in the Upper Jurassic Dupuy Formation, the Lower Cretaceous Malouet Formation, Flacourt Formation and Tunney Member, Mardie Greensand Member and M zones of the Muderong Shale and in the Upper Cretaceous Gearle Siltstone. Approximately 850 wells have been drilled to appraise and develop these accumulations, and to provide water source and water injection wells to enhance recovery. Production commenced in December 1966, with the first shipment of oil in April 1967. Although numerous hydrocarbon reservoirs have been developed, 95% of the 44 GL (278 MMBBL) of produced oil has been from the Windalia Sand.Structural development of the Barrow Island anticline occurred initially during the Middle Jurassic and continued intermittently during the Cretaceous and Tertiary. Initial charging of the Dupuy and Malouet formations with oil from the Upper Jurassic Dingo Claystone occurred in the Early Cretaceous prior to the development of the shallower closures. Periodic wrench- related movement on the Barrow Fault during the Early to Late Cretaceous produced closures at the Lower Cretaceous reservoirs and provided a catalyst for oil migration and charging of these closures. Significant amounts of an extremely biodegraded component, and several less biodegraded phases are present in the oil in the Windalia Sand, indicating several phases of oil charging of the Barrow structure from Middle and Upper Jurassic sediments. In the Tertiary, gas sourced from Triassic and Jurassic sediments migrated into the Barrow structure via a dilated Barrow Fault, charged the Middle Jurassic Biggada Formation and displaced some of the oil in the Lower Cretaceous reservoirs.

Sharks from the mid-Cretaceous Gearle Siltstone, Southern Carnarvon Basin, Western Australia

ABSTRACT A small but diverse selachian fauna, represented by isolated teeth, is described from the lower part of the Gearle Siltstone in the Giralia Anticline, Southern Carnarvon Basin, Western Australia. It consists of: Notorynchus aptiensis, Paraorthacodus sp., Orectolobiformes gen. et sp. nov., Archaeolamna sp., Leptostyrax sp., Cretoxyrhinidae gen. et sp. incertae sedis, Cretolamna sp., Paraisurus aff. compressus, Carcharias striatula, and Squalicorax primaevus. Comparisons with known ranges of these taxa in the Northern Hemisphere place the fauna in the middle to late Albian. Contrary to previous claims, the cowshark Notorynchus aptiensis does not appear to have been primarily a Tethyan faunal element, as its teeth are more common in the cool water facies of the Gearle Siltstone than anywhere else in the world. The few previous records of Cretaceous sharks from Western Australia are revised, resulting in several systematic alterations.

PETROLEUM PLAY ELEMENT PREDICTION FOR THE CRETACEOUS-TERTIARY BASIN PHASE, NORTHERN CARNARVON BASIN

A regional tectono-stratigraphic framework has been developed for the Cretaceous and Tertiary section in the Northern Carnarvon Basin. This framework places traditional observations in a new context and provides a predictive tool for determining the temporal occurrence and spatial distribution of the lithofacies play elements, that iss reservoir, source and seal.Two new, potential petroleum systems have been identified within the Barremian Muderong Shale and Albian Gearle Siltstone. These potential source rocks could be mature or maturing along a trend that parallels the Alpha Arch and Rankin Platform, and within the Exinouth Sub-basin.A favourable combination of reservoir and seal can be predicted for the early regressive part of the Creta- ceous-Tertiary basin phase (Campanian-Palaeocene). Lowstand and transgressive (within incised valleys) reservoirs are more likely to be isolated and encased in sealing shales, similar to lowstand reservoir facies deposited during the transgressive part of the basin phase, for example, the M. australis sand play.The basin analysis revealed the important role played by pre-existing Proterozoic-Palaeozoic lineaments during extension, and the subsequent impact on play elements, in particular, the distribution of reservoir, fluid migration, and trap development. During extension, the north-trending lineaments influenced the compart mentalisation of the Northern Carnarvon Basin into discrete depocentres. Relay ramp-style accommodation zones developed, linking the sub-basins, and acting as pathways for sediment input into the depocentres and, later in the basin's history, as probable hydrocarbon migration pathways. The relay accommodation zones are a dynamic part of the basin architecture, acting as a focal point for response to intraplate stresses and the creation, modification and destruction of traps and migration pathways.

Mid-Cretaceous calcareous and siliceous microfossils from the basal Gearle Siltstone, Giralia Anticline, Southern Carnarvon Basin

A diverse assemblage of foraminifera, radiolaria, and calcareous nannoplankton is recorded from basal beds of the Gearle Silstone in its type area. The calcareous nannofossils place the strata within the temperate-latitude CC8a Subzone and the Southern Ocean Sollasites falklandensis Subzone; the planktonic foraminifera indicate that the beds belong to the Hedbergella planispira Zone of the Australian region. The biozones suggest an early Albian age (later than the earliest Albian). Abundant calcareous and siliceous plankton and the Marssonella Association of benthonic foraminifera indicate normal-marine depositional conditions in a water depth of about 100 m. The transition from Windalia Radiolarite to Gearle Siltstone may reflect a marine transgressive pulse that reactivated bottom-water circulation and facilitated a significant increase in the calcareous biogenic component of the sediment. Deposition of the basal Gearle Siltstone was coincident with a major increase in bathymetry in the Papuan, Laura, C...

Cool Cretaceous climates: new data from the Albian of Western Australia

Although previous geological, palaeobotanical and geochemical data from the Albian of Australia have suggested the possibility of cool palaeoclimates, there are few well-constrained stable isotope palaeotemperatures studies from this region. In this paper we present the results of stable isotope analyses of dimitobelid belemnites from the Albian Gearle Siltstone, Carnarvon Basin, Western Australia. Oxygen isotope values for diagenetically least-altered samples give values ranging between δ 18 O −0.02 and 0.85% PDB, which equate to a mean palaeotemperature of 10.08°C (assuming δ w of −1.2% SMOW). This value is broadly consistent with recently published isotope data from the Albian Antarctic and with estimates for mid-Cretaceous latitudinal temperature gradients, implying that cool palaeoclimates occurred during a period of 'Greenhouse' Earth.