Shallow Marine Strata Research Articles

Marine volcaniclastics of the Hidden Lake Formation (Coniacian) of James Ross Island, Antarctica: an enigmatic element in the history of a back-arc basin

Abstract The Coniacian Hidden Lake Formation of James Ross Island, Antarctica is a 300–400 m-thick succession of marine volcaniclastic conglomerates, sandstones and mudstones. It occurs at a point of transition in the evolution of the James Ross Basin, as it is underlain by deep-marine strata and overlain by shallow-marine strata. The succession reflects the two main factors controlling the deposition of the formation: (1) the influx of large quantities of volcaniclastic sediment; and (2) a pronounced inversion event in the early Coniacian heralding the cessation of transpressive tectonic activity in the James Ross Basin. The succession is dominated by a range of sediment density-flow deposits, which, combined with the limited faunas and the lack of wave-induced structures, suggest deposition in a relatively deep-marine environment below storm-wave base. Three main facies associations are recorded representing base-of-slope, fan-delta and basin-floor depositional environments. The volcaniclastic fan-delta association is dominated by fresh pyroclastic detritus and was deposited in response to volcanic eruptions on the adjacent arc. Thick beds of parallel-stratified sandstone record deposition from sustained, concentrated sediment density flows. The conditions immediately following pyroclastic eruptions lend themselves to the deposition of such deposits, as vegetation cover is destroyed and large amounts of poorly consolidated sediment are available for reworking. An enigmatic feature of the succession is the presence of units of cross-bedded sandstones thought to be of tidal origin that are locally abundant and are intimately interbedded with sediment density-flow deposits. The occurrence of tidal sediments in a substorm-wave base setting is explained by appealing to partial basin inversion during the final phases of strike-slip tectonic activity in the basin creating an irregular basin floor that focused and amplified tidal currents. The infilling of this basin topography by sediment and waning intrabasinal tectonism during the Coniacian resulted in the progressive elimination of this basin-floor topography and the onset of shallow-marine shelf sedimentation.

Turonian (Upper Cretaceous) lithostratigraphy and biochronology, southern Gulf Islands, British Columbia, and northern San Juan Islands, Washington State

Clastic strata preserved on Sidney Island, Barnes Island, and adjacent islands of the southernmost Gulf Islands of British Columbia and the northern San Juan Islands of Washington State are assigned to new stratigraphic units: the Sidney Island Formation and the Barnes Island Formation. The Sidney Island Formation consists of basal conglomerate and sandstone that grades upward through planar-stratified sandstone into hummocky cross-stratified sandstone and siltstone, all of which are deposited in relatively shallow-marine environments. The Barnes Island Formation, in contrast, consists of deep-marine conglomerate, sandstone, and mudstone that was deposited in a submarine-fan setting. Mollusk fossils from the Sidney Island Formation are of Early to Middle Turonian age, whereas ammonites and foraminifers from the Barnes Island Formation indicate a Late Turonian age. The Sidney Island Formation thus records initial marine transgression and inundation of basement rocks, followed by basin deepening that is transitional to the deep-marine submarine-fan deposits of the Barnes Island Formation. Sidney Island Formation strata have been considered previously as derived from uplift along the nearby San Juan thrust system in mid-Cretaceous time. However, the shallow-marine strata are internally well organized, and the facies succession is persistent across the formation's outcrop area. In addition, the formation lacks the distinctive detrital metamorphic mineral assemblages that are characteristic of older rocks of the San Juan Islands. These observations suggest that strata of the Sidney Island Formation did not accumulate immediately adjacent to active thrusting but rather in a more distal setting relative to the thrust system.

PERMIAN AMMONOIDS PROSTACHEOCERS AND PERRINITES FROM THE SOUTHERN KITAKAMI MASSIF, NORTHEAST JAPAN

Permian shallow marine strata are widely distributed in the Southern Kitakami Massif, Northeast Japan. They yield many marine fossils and have been well studied both stratigraphically and biostratigraphically. However, the geological age of the upper part of the Permian in the Tassobe district, located in the northern part of the Southern Kitakami Massif (Fig. 1), is still an unsettled question. The Permian is divided into the lower Tassobe and upper Sotokawame Formations (Okuyama, 1980; Yoshida et al., 1992). Early Permian (Sakmarian–Artinskian) fusulinoideans have been known from the middle to lower-upper part of the Tassobe Formation (Hirokawa and Yoshida, 1956; Saito, 1968; Yoshida et al., 1992), whereas no age-diagnostic fossils have been found from the overlying Sotokawame Formation. In the course of the geologic survey of the Permian, we collected two ammonoid fossils from the lower part of the Sotokawame Formation, which can provide us a basis for stratigraphic correlation. This paper describes these ammonoids and discusses their stratigraphic and biogeographic significance. Figure 1 —Location map of the study area The Carboniferous Okawame and Permian Tassobe and Sotokawame formations occupy the Tassobe district (Fig. 2). The Tassobe Formation, unconformably covering the Lower-Middle Carboniferous Okawame Formation, is divided into three lithologic units: the lower, middle, and upper parts. The lower part is 300– 350 m thick and consists of sandstone and mudstone, with minor amounts of conglomerate (Fig. 3). The middle part, 450–550 m in thickness, is composed mostly of limestone with intercalated mudstone. The upper part consists mainly of mudstone associated with lenticular limestones and its total thickness reaches 450 m. From the limestone of the middle part, fusulinoidean fossils such as Robustoschwagerina shellwieni (Hanzawa, 1938); Pseudofusulina kraffti (Shellwien and Dyhrenfurth, 1909); P. paramotohashii Morikawa, 1960; Charaloschwagerina vulgaris (Shellwien and Dyhrenfurth, 1909); and Parafusulina chihsiaensis (Lee, 1931 …

A New Ichnospecies of Arthrophycus from the Upper Cambrian-Lower Tremadocian of Northwest Argentina: Implications for the Arthrophycid Lineage and Potential in Ichnostratigraphy

A new ichnospecies of Arthrophycus Hall 1852, A. minimus , is described from Upper Cambrian-Lower Tremadocian, shallow-marine strata of northwest Argentina. This new ichnospecies consists of small, long, regularly annulated hypichnial elements displaying subcircular to squarish cross-section and a ventral median groove. Side branches are occasionally present, but palmate, fan-like structures and scribbling patterns are absent. We adopt a relatively narrow diagnosis of Arthrophycus , suggesting that roughly annulated, cylindrical structures should not be included in this ichnogenus, unless other diagnostic features (i.e., squarish cross-section, median groove, zipper-like annulations) are also present. Arthrophycus is a common ichnotaxon in Ordovician-Silurian shallow-marine siliciclastic environments. Post-Paleozoic occurrences are removed from Arthrophycus . Arthrophycus has been proposed as a biostratigraphic index fossil in Ordovician-Silurian rocks. The presence of A. minimus in the Santa Rosita Formation of northwest Argentina indicates that Arthrophycus ranges at least from the Upper Cambrian-Lower Tremadocian with probable representatives in the Lower Cambrian and, therefore, its biostratigraphic utility is extended. Arthrophycus minimus represents the first Cambrian occurrence exhibiting not only fine, diagnostic morphologic features, but also the classical Arthrophycus behavioral pattern in dense monoichnospecific assemblages. The exploratory behavioral pattern displayed by A. minimus is simpler than that of the younger ichnospecies, particularly A. brogniartii, A. alleghaniensis, and A. lateralis . This is consistent with the basal position of A. minimus within the arthrophycid lineage.

The Clearwater Formation, Cold Lake, Alberta: a worldclass hydrocarbon reservoir hosted in a complex succession of tide-dominated deltaic deposits

Research Article| September 01, 2002 The Clearwater Formation, Cold Lake, Alberta: a worldclass hydrocarbon reservoir hosted in a complex succession of tide-dominated deltaic deposits G. Glen McCrimmon; G. Glen McCrimmon Exxon-Mobil Exploration Co., P.O. Box 4778, Houston, Texas 77120, USA Search for other works by this author on: GSW Google Scholar R.W.C. Arnott R.W.C. Arnott Department of Earth Sciences and Ottawa-Carleton Geoscience Centre, University of Ottawa, Ottawa, ON K1N 6N5 Search for other works by this author on: GSW Google Scholar Author and Article Information G. Glen McCrimmon Exxon-Mobil Exploration Co., P.O. Box 4778, Houston, Texas 77120, USA R.W.C. Arnott Department of Earth Sciences and Ottawa-Carleton Geoscience Centre, University of Ottawa, Ottawa, ON K1N 6N5 Publisher: Canadian Society of Petroleum Geologists Received: 15 Oct 2001 Revision Received: 26 May 2002 First Online: 02 Mar 2017 Online ISSN: 2368-0261 Print ISSN: 0007-4802 © The Society of Canadian Petroleum Geologists Bulletin of Canadian Petroleum Geology (2002) 50 (3): 370–392. https://doi.org/10.2113/50.3.370 Article history Received: 15 Oct 2001 Revision Received: 26 May 2002 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation G. Glen McCrimmon, R.W.C. Arnott; The Clearwater Formation, Cold Lake, Alberta: a worldclass hydrocarbon reservoir hosted in a complex succession of tide-dominated deltaic deposits. Bulletin of Canadian Petroleum Geology 2002;; 50 (3): 370–392. doi: https://doi.org/10.2113/50.3.370 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of Canadian Petroleum Geology Search Advanced Search Abstract The Lower Cretaceous (Lower Albian) Clearwater Formation at Cold Lake, Alberta is a complex assemblage of nonmarine, marginal marine and shallow marine strata. Within the study area, these strata form four unconformity-bounded depositional sequences that in most cases consists of a lower progradational unit overlain by a retrogradational unit. The base of all sequences, except the first, is marked locally by a deeply incised, northwest–south-trending paleovalley. Although eroded during a fall of relative sea level, each paleovalley was filled mostly with sediment deposited by a northwest-prograding tide-dominated delta during each subsequent rise of relative sea level. Reservoir strata occur in the lower, deltaic part of the three youngest sequences, and consist primarily of nonmarine fluvial and high-energy tidal sand flats deposits. Deltaic progradation was halted, however, by a possible decrease in sediment flux and a return to retrogressive conditions. This formed an areally extensive flooding surface, which then was overlain by retrogradational shallow-marine deposits (embayment-mouth deposits). Accordingly, each sequence represents a short-term (4th order?) fluctuation of relative sea level that was superimposed on a long-term (3rd order?) rise of relative sea level. In addition, each successive sequence indicates a progressively more basinward migration of (tide-dominated) deltaic facies (prograding sequence set) suggestive of temporal changes in some number of local and/or regional sedimentary controls, such as sediment flux, rates of relative sea level, or depositional gradient. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Unusual coarse, clastic, wave-dominated shoreface deposits, Pliocene to middle Pleistocene, northern Chile; implications for coastal facies analysis

ABSTRACT Pliocene to mid-Pleistocene shallow-marine strata exposed at Gatico, northern Chile (22° 30 S, 70° 14 W) display unusual characteristics for coarse-grained wave-dominated shoreface deposits. A 15 m2000 m strike section of Pliocene to mid-Pleistocene strata exposed within small, late Pleistocene sea cliffs contain four facies: beach, foreshore channel, upper shoreface, and rip channel. Beach and upper shoreface facies predominate. The latter consists of locally bioturbated, coarse-grained, pebbly sandstone with (1) single sets of planar to tangential cross-strata, up to 3.5 m thick, with reactivation surfaces, and (2) sets of medium-to large-scale cross-strata that were deposited on inclined set surfaces (up to 15°). Some of the inclined set surfaces extend up-dip of cross-set terminations. Inclined parallel laminae and bioturbated beds lie subparallel to the inclined set surfaces. Interpretation of these unusual bedforms has been made through analogy with the present-day microtidal wave-dominated coastline at Gatico, by facies analysis, and by comparison with modern-day wave-dominated shorelines. The large-scale sets of cross-strata record the migration of simple, two-dimensional (2-D) dunes and represent the slipface deposits of nearshore bars. Parallel laminae represent the bar surface. Medium-scale dunes migrated up the inclined set-bounding surfaces, which lie subparallel to older preserved bar surfaces. Paleocurrent data indicate that bar migration was largely directed obliquely onshore towards the southeast. The bar slipface deposits are considered to record the recovery of a nearshore bar immediately following storm conditions. Bar recovery involves re-establishment of a slipface and rapid onshore migration. Erosion surfaces that appear as the extension of lower set boundaries beyond the termination of cross-strata sets, and reactivation surfaces, are considered to be storm-generated. The episodic nature of bar migration is indicated by the development of heavily bioturbated horizons. The small-scale ascending cross-sets may reflect fair-weather conditions with northwestward dune migration up the modified bar slipface. Despite the abundance of barred shorelines around the world at present, their deposits are not widely described from the rock record, suggesting that they are either not preserved or not recognized. The scale and internal characteristics of these nearshore bars closely resemble those of some tide-generated bedforms (e.g., compound cross-strata in marine sediment with bimodal-bipolar paleocurrents and evidence of episodic bedform migration). The similarity of these north Chilean deposits to documented tidal deposits suggests that an element of caution may be required in the identification of tidal deposits from the rock record.

Regional Structural Style of the Central and Southern Oman Mountains: Jebel Akhdar, Saih Hatat, and the Northern Ghaba Basin

ABSTRACT Three quantitative regional transects across the Saih Hatat and Jebel Akhdar anticlines in the Central and Southern Oman Mountains and the Northern Ghaba Basin have been constructed based on surface, well and seismic data. Interpreted large-scale structural geometries suggest that the Saih Hatat and Jebel Akhdar anticlines are basement-involved compressional structures, underlain by north-dipping, high-angle, blind, reverse faults located beneath their southern limbs. A compressional deformation event initiated in the Oligocene (constrained by apatite fission track data) involving the high-angle reverse faults is interpreted in which pre-Permian strata and Permian-through-Lower Cretaceous strata, exposed in the Saih Hatat and Jebel Akhdar anticlines, were parautochthonous - uplifted over the underlying reverse faults, and not displaced a great distance laterally. The allochthonous Hawasina and Sumeini sedimentary rocks and the Semail Ophiolite complex are interpreted to have been emplaced onto the carbonate platform during the Late Cretaceous, and have subsequently been parautochthonous during the Tertiary deformation. The upper portion of the pre-Permian section in the Ghaba Basin consists predominantly of a thick (>4 kilometers) sequence of Cambrian-through-Silurian, predominantly non-marine to shallow-marine, clastics of the Haima Supergroup. In contrast, out of the Ghaba Basin proper in the Central Platform or Musallim High region, the Haima Supergroup is generally less than 2 kilometers thick, and interpreted to thin to the north. The fundamental difference in pre-Permian strata exposed in the Saih Hatat and Jebel Akhdar Anticline windows is the thick (>3.4 kilometers) section of Ordovician age, shallow-marine strata (Amdeh Formation) present in the Saih Hatat Anticline, but absent in the Jebel Akhdar Anticline. In our interpretation, the shallow-marine clastics exposed in the Saih Hatat Anticline represent the northern extension of the Early Paleozoic Ghaba Basin, which have been uplifted over a high-angle reverse fault in the Early Tertiary deformation event. The cross-section through Jebel Akhdar is located to the northwest of the Ghaba Salt Basin, along the Musallim High. In this area the thickness of the Ordovician strata deposited is interpreted to be less than in the Ghaba Basin. The Ordovician section is not present in the Jebel Akhdar structure - the thinned section likely eroded in a Late Paleozoic deformation event.

Have local stages outlived their usefulness for the New Zealand Pliocene‐Pleistocene?

Stratigraphic subdivision of the New Zealand Pliocene‐Pleistocene has its historical basis in biostratigraphy, especially in the shallow marine strata in the Wanganui and East Coast basins, North Island. Internationally, an astronomically calibrated, high resolution timescale is now well established for the Pliocene‐Pleistocene, based on predominantly precession‐controlled sedimentary cycles in the Mediterranean region, and the mainly obliquity‐controlled, high resolution, deep‐sea, benthonic oxygen isotope record. Naish and others have recently provided a similar Milankovitch‐frequency cyclostratigraphy for the Pliocene‐Pleistocene of New Zealand (last 3.6 m.y.), based on the Wanganui Basin record. This cyclostratigraphic record allows numeric ages to be estimated for at least 114 stratigraphic horizons in Wanganui Basin, which are not otherwise dated. Based on this record, we propose new definitions for the boundaries between New Zealand Pliocene‐Pleistocene stages. The new boundaries respect traditional biostratigraphic indicators, but are based upon objective markers, mainly magnetic polarity reversals. The two exceptions are the base‐Marahauan (equated with the international Pliocene‐Pleistocene boundary, at 1.81 Ma) and the base‐Haweran (equated with the base of the Rangitawa Tephra, at 0.34 Ma). The revised positions and ages for the age/stage and subage/substage boundaries, from older to younger, are: base‐Opoitian Stage, base‐Thvera subchron, 5.25 Ma; base‐Waitotaran Stage (= base‐Waipipian Substage), Gauss/Gilbert boundary, 3.60 Ma; base‐Mangapanian Substage, top‐Kaena Subchron, 3.03 Ma; base‐Nukumaruan Stage (= base‐Hautawan Substage), Matuyama/Gauss boundary, 2.58 Ma; base‐Marahauan Substage, Pliocene‐Pleistocene boundary, 1.81 Ma; base‐Castlecliffian Stage (= base‐Okehuan Substage), base‐Jaramillo Subchron, 1.07 Ma; base‐Putikian Substage, Brunhes/Matuyama boundary, 0.78 Ma; base‐Haweran Stage, base‐Rangitawa Tephra, 0.34 Ma.

Integration of Sequence Stratigraphy and Fault Population Studies in Integrated Reservoir Modelling Software for Field Development and Management: ABSTRACT

Comprehensive datasets of a subsurface Carboniferous coastal plain succession in Alaska and large scale outcrops of Cretaceous shallow marine strata of the Book Cliffs, Utah, have been used in the development of 3-D reservoir modelling software which has functionality allowing modelling of sedimentary architectural elements, key stratigraphic surfaces, seismic and sub-seismic faults and well/seismic data. Application of high resolution sequence stratigraphic concepts allows deterministic correlation of flooding surfaces and sequence boundaries, prediction of spatial/temporal settings of well- and poorly-connected sands and guidelines for interpretation of channel dimensions. Initial models allow 3-D visualisation of the reservoir and facilitate a better understanding of the distribution of heterogeneities. Subsequent assignment of attributes such as facies and permeability allow calculations and operations (e.g connectivity) to be carried out on the model. The frequency and geometry of seismically resolved faults is used to predict likely numbers and sizes of sub-seismic faults, which may impact reservoir performance and compartmentalisation. Calculation of the deformation of the sequence architecture by sub-seismic faults and refinement of the cellularisation methods allows investigation of the juxtaposition and fault seating effects on fluid flow. The modelling system permits quantitative analysis of the relative importance of complex depositional architecture and subseismic faults onmore » reservoir connectivity. However, the results of flow modelling are strongly dependent on the resolution of cellular models in relation to the scales of stratigraphic and structural features.« less

Quantification of Reservoir Geometries and Facies Proportions in Wave-Dominated, Shallow Marine Sandstones: ABSTRACT

To reliably appraise shallow marine hydrocarbon reservoirs, it is important to have a quantitative understanding of the geometries of parasequences, and the spatial distribution of facies within them. This study discusses the quantification of parasequence geometries and intraparasequence facies proportions in wave-dominated, shallow marine strata exposed in large scale outcrops in the Book Cliffs, Utah. The Grassy Member of the Blackhawk Formation consists of two, wave-dominated shallow marine sequences. Sequence 1 of the Grassy Member comprises two parasequences. Analysis of the depositional architecture of Parasequence 1 and Parasequence 2 shows they have very different geometries. Parasequence 1 is an aggradational-progradational parasequence which thickens gradually basinward. In contrast, Parasequence 2 is strongly progradational and shows and abrupt thickening in a basinward direction. Although Parasequence 1 and 2 have different dipsection geometries, analysis of changes in shoreface facies proportions in both, shows they have similar trends in the overall distribution of facies from proximal to distal positions. The Book Cliffs outcrops enable a complimentary analysis of strike geometries and facies proportions to also be undertaken. This study shows that within two progradational parasequences which have different geometries, there is a degree of predictability in the overall distribution of the wave-dominated shallow marinemore » shoreface facies. This suggests that, with an understanding of the large-scale depositional architecture of wave-dominated sandstones in the subsurface, the overall changes in the proportion of shoreface facies within that system can be predicted.« less

Late Ordovician evolution of the intracratonic basins in north-west Gondwana

The Palaeozoic intracratonic basins in northwest Gondwana, i.e. the Amazonas, Parnaiba and Acera basins, probably opened during late Caradoc and Ashgill times. The fluviatile sedimentation later changed to littoral at the basinal margins. A transgression from the north-west region of Gondwana slowly overlapped the margins of the intracratonic basins. The transgression reached its maximum in the Rawtheyan (late Ashgill), as evidenced by fossiliferous shallow marine sediments in the Amazonas Basin. The Hirnantian glaciation in north Gondwana lowered the sea level, and in the Amazonas Basin a littoral sedimentation followed on shallow marine strata. From the opening of the basins onwards, a shallow sea probably existed close to the epicontinental basins in north-west Gondwana. The basins were connected via a narrow passage between the Guayana and Ivorian cratons.

Longshore-Drift Dispersed, Storm-Generated Cross-Stratified Sandstone from Some Cretaceous Shallow Marine Strata, Rocky Mountain Region: ABSTRACT

Most Cretaceous shallow marine strata of the Rocky Mountain region are characterized by asymmetrical upward-coarsening and upward-thickening sequences. The strata typically contain similar lithofacies (i.e., normally graded planar parallel laminated claystone, siltstone, and sandstone; hummocky cross-stratified sandstone; symmetrical and asymmetrical ripple cross-lamination; and trough and planar tublar cross-stratified sandstone) and display an upward increase in the thickness and frequency of sharp-based sandstone beds that grade into amalgamated cross-stratified sandstone. Most workers agree that sharp-based sandstone beds and hummocky cross-stratified sandstone are storm generated. However, the origin of trough and planar tabular cross-stratified sandstone is controversial. Most workers interpret these sedimentary structures as deposited from either storm-generated traction currents or combined flow currents on the shelf, tide-generated traction currents, or tidally dispersed, storm-generated suspension clouds. Detailed analysis of three-dimensional outcrops has revealed several significant features of these sedimentary structures that indicate they may have been deposited by longshore drift dispersed, storm-generated suspension clouds. Sets of trough and planar cross-stratified sandstone form medium-scale discontinuous, irregularly shaped sand bodies, bound by erosional surfaces and composed of unidirectional dip-oriented cross strata. Individual cross stratum commonly have a sigmoidal shape, are bound by either reactivation surfaces or mudstone drapes, and contain normally graded concordant laminae.more » Top-set laminae, are truncated by the upper set boundary, whereas bottom-set laminae, become asymptotic to the lower set boundary and commonly are reworked and overlain by wave generated, ripple cross-lamination or mudstone drapes.« less

Paleo-Reconstruction of Shelf-Slope Margin Along San Emigdio Mountains, Southern San Joaquin Valley, California: ABSTRACT

Deformation along the San Emigdio Mountains, California, is characterized by large overthrust sheets that carried Eocene through Miocene nonmarine to shallow marine strata over their deeper marine equivalents. The Pleito thrust has at least 20,000 ft of throw and is the major structural feature of this overthrust belt. The upper plate of the Pleito thrust carries an extensively exposed block of three prograding sequences and, along strike, partially exposes the shelf-slope boundaries of these units. Equivalent changes are observed in the subsurface beneath the overthrust. Total crustal shortening along this region ranges from 25 to 50%, with most of the shortening taken up by the Pleito thrust. The thrust has a low-angle, ramp-and-glide configuration, but on the south, the strata eventually extend downward into basement. The northern boundary to the deformation belt is the White Wolf and Pioneer fold and thrust structures. These features form a transfer zone, where one structure ends and the other feature begins. The amplitude of these folds can be up to 10,000 ft from crest to trough. Because of the large overthrust of the Pleito thrust, reservoir rocks are found up to 7 mi south of the fault's surface trace in the lower plate. Themore » buried strata lie south and west of producing fields along the White Wolf fault and the Tejon embayment. The complicated stratigraphic changes combined with the thrust-belt structures require retrodeformable crustal profiles to take into account the facies distributions to model the hydrocarbon potential of this lightly explored province.« less

Provenance of Conglomerate Clasts from Upper Cretaceous Kuskokwim Group, Southwest Alaska: ABSTRACT

The predominantly Upper Cretaceous (Albian to Coniacian) Kuskokwim Group consists of marine turbidites and subordinate fluvial and shallow marine strata, deposited in an elongate southwest-trending basin covering over 70,000 km2 in southwestern Alaska. In the Sparrevohn and Cairn Mountain areas of the Lime Hills A-7 and A-8 quadrangles, fluvial, inner fan, middle fan, and outer fan facies are stacked with distal facies over proximal facies by northwest vergent thrust faults. Inner fan pebble-to-cobble conglomerate and pebbly sandstone were deposited as submarine grain flows up to 10 m thick containing reversely graded bases and normally graded tops. Clasts from these conglomeratic deposits are predominantly sedimentary rock fragments--particularly sandstone, si tstone, and argillite--originally thought to have been derived from the nearby Jurassic and Lower Cretaceous flysch (Kihiltna terrane). Detailed examination of these clasts, however, indicate that they contain as minor constituents Paleozoic coral, oolitic limestone, algal boundstone, radiolarian chert, and mafic, intermediate, and felsic volcanic rocks, most likely derived from the adjacent Nixon Fork, Dillinger, and Mystic terranes. Sandstone clasts are arkosic (Q25-F37-L38, n = 7) and contain subequal amounts of K-feldspar and plagioclase. Sand grains within these clasts are moderately sorted, subrounded, and have presolved contacts. Similar arkosic rocks have been described from the Dillinger terrane of the McGrath quadrangle and are the most likely source for the pebbles and cobbles within the Kuskokwim Group. End_of_Article - Last_Page 660------------

Origin and Early History of Mollusks

Fossil mollusks are common and conspicuous objects in shallow marine strata of Ordovician and younger ages, and so the broad features of the post-Cambrian history of the Mollusca have been known for many years. However, as all but one of the eight classes of the Mollusca had evolved by the beginning of the Ordovician, it is obvious that the pre-Ordovician history of the Mollusca holds the key to an understanding of the primary radiation of the phylum. It was with this view in mind, that John Pojeta, Jr. and I began our joint studies of Cambrian mollusks some 13 years ago.

New Zealand Pleistocene and late Pliocene glacio-eustatic cycles

A correlation diagram relates the numbered oxygen isotope stages of Shackleton and Opdyke's oxygen isotope curve for core V28–239, as extended by Gardner, to Nukumaruan, Castlecliffian and younger sedimentary cycles in Hawke's Bay, southern Wairarapa, and Wanganui districts and to the flight of marine terraces (Castlecliffian and younger) recognised by Pillans near Wanganui. Local correlation is principally by biostratigraphy, but also by lithostratigraphy and fission-track dates on tephras. Important calcareous nannofossil datums for both local and international correlation are: (1) FAD of Gephyrocapsa sinuosa in the mid-Nukumaruan of the New Zealand region, both in DSDP site 284, just above the (base Olduvai) LAD of Discoaster brouweri, and in three on-land sequences (within the Olduvai at Mangaopari Stream); (2) LAD of Pseudoemiliania lacunosa, during isotope stage 12, last known in the Castlecliffian Upper Kai-Iwi Siltstone at Wanganui; and (3) FAD of Emiliania huxleyi during isotope stage 8, and recorded tentatively from the Waipuna Delta Conglomerate (Ngarino Terrace cover beds; Hawera Series) near Wanganui. Magnetic polarity is also used for external correlation. Most of the prominent oxygen isotop events in core V28–239 are inferred in New Zealand on-land sequences (about twenty cycles). The Wanganui area demonstrates an apparently unique occurrence of coeval marine terrace deposits and offshore marine rocks representing all major high sea levels between isotope stages 11 and at least 17. Nukumaruan sedimentary cycles recognised in eastern North Island are relatively complete, most inferred glacial periods being represented by shallow marine strata. In contrast, Castlecliffian and younger marine sedimentary cycles in southwestern North Island are less complete, each inferred interglacial deposit being bounded by disconformities. The difference between Nukumaruan and younger sedimentary cycles possibly supports the hypothesis of an increase in the amplitude of sea-level change about a million years ago. The Pliocene/Pleistocene boundary lies above the middle of the New Zealand Nukumaruan Stage.

Tectonic and Climate Controls on Sedimentation of Upper Miocene Nonmarine Strata East of San Francisco Bay, California: ABSTRACT

Upper Miocene nonmarine and shallow-marine strata exposed east of San Francisco Bay record a change from convergent-margin to transform-margin tectonics. During the late Miocene, the East Bay area occupied the oceanward side of a shelved fore-arc basin which was progressively incorporated in the evolving San Andreas strike-slip orogene. Patterns of deposition in the broad fore-arc basin were relatively simple: andesitic arc-derived detritus was transported the full width of the fore-arc basin from the Sierras to the East Bay area. In contrast, the wrench-tectonic regime produced complex patterns of sedimentation displaying greater local variability. Based on stratigraphic data from D. Wagner and our observations, we infer that the west-facing slope of the fore-arc basin i the East Bay area was reversed about 13 m.y. ago, with uplift of the area between the eventual traces of the San Andreas and Hayward faults. A fluvial clastic wedge was shed eastward into the East Bay area from this uplifted terrane of Mesozoic subduction-complex and fore-arc basin rocks. Initial rupturing along a proto-Hayward fault zone followed the uplift about 10 m.y. ago. Loci of basaltic volcanism (10 to 7 m.y.) along these fractures interfinger with the clastic wedge. A similar pattern of uplift and drainage reversal apparently heralded the onset of wrenching along the Calaveras trend 6 to 8 m.y. ago. Expansion of the strike-slip orogene segmented the outer fore-arc basin into local basins, some characterized by periodic lacustrine deposition and probable internal drainage. By he end of the Miocene, Sierran arc volcanism waned at the latitude of San Francisco Bay, and arc-derived volcaniclastics were fully supplanted by recycled Coast Range-derived detritus in the East Bay area. Certain of these Coast Range sediment sources, particularly blueschist-bearing Franciscan terranes, serve to constrain estimates of strike-slip on the Hayward fault. Although tectonics provide the principal control on patterns and composition of Neogene sedimentation in the East Bay area, late Miocene climatic fluctuations left a marked overprint. Red beds, typical of the lower upper Miocene and suggestive of seasonal aridity, correlate with a relatively cold period along the northeast margin of the Pacific Ocean. In contrast, overlying middle upper Miocene fluvial-lacustrine beds are gray brown and contain abundant carbonaceous debris and faunas indicative of a marked decrease in aridity. These strata correspond to a period of warming in the later Miocene prior to latest Miocene refrigeration (Messinian). End_of_Article - Last_Page 1689------------

Stratigraphy and sedimentology of the Coalgate area, Canterbury, New Zealand

Abstract Stratigraphic and sedimentologic studies of the Cretaceo-Tertiary sequence overlying Torlesse rocks in the Coalgate area permit a review of the lithostratigraphy of the region. Much new information has come from borehole logs and cores. Coal measures conformably overlain by shallow marine strata and associated igneous rocks, including View Hill Basalt, are assigned to the Eyre Group (modified name) of upper Cretaceous and possibly Paleocene age. Burnt Hill Group (new name) is erected to accommodate some 200 m of Oligocene-Miocene non-calcareous marine, estuarine, and associated igneous rocks of central and northern Canterbury. Formations include: Homebush Standstone (new name for a massive estuarine deltaic deposit) (oldest unit); Thongcaster Formation of Duntroonian age (new name for a fossiliferous clayey fine sandstone); Wairiri Volcaniclastite (new name for a tuff-breccia deposit); Chalk Quarry Sandstone of Waiauan age (modified name); Chalk Hill Clay; Sandpit Tuff (name now formalised) consi...