Major Depositional Episodes Research Articles

Stratigraphic organisation of the Jurassic sequence in Interior Oman, Arabian Peninsula

ABSTRACT A Stratigraphic model is proposed for the Jurassic sequence in Interior Oman. The model is based on regional well-log correlations, outcrop analysis and integration of Biostratigraphy. Large-scale architectures are restored using a well-to-well correlation technique, after the well-log markers of the relevant surfaces of sequence stratigraphy are identified. This identification is achieved by comparing well-log signatures to lithological and sedimentological columns of nearby exposed sections. The subsurface dataset consists of 19 wells arranged in two east-west profiles, 341 km and 332 km long. The Jurassic sequence in Interior Oman shows a general easterly thinning wedge and includes two hiatuses with marked age-gaps. Three major depositional episodes are identified: (1) a Pliensbachian-Toarcian coastal encroachment in a southward direction, represented by the dominantly clastic deposition of the Lower Mafraq Formation upon the Permian carbonates; (2) a general late Bajocian marine flooding (hybrid facies of marginal-marine environments of the Upper Mafraq Formation), followed through the Bathonian-Callovian by the carbonate Dhruma-Tuwaiq System which evolved through time from a low-angle, homoclinal ramp dipping in a (north) westwards direction, to a purely aggradational, flat-topped platform (upper Dhruma and Tuwaiq Mountain formations); (3) a Kimmeridgian-Tithonian onlap in an eastwards direction of finegrained limestones (Jubaila-Rayda) upon the post-Tuwaiq unconformity. Depositional hiatuses in the early Liassic and at the Early-Middle Jurassic transition are likely to reflect major eustatic sea-level lowstands. In contrast, subsurface correlations of the MFSs through the Dhruma-Tuwaiq indicate that the post-Tuwaiq unconformity is a low-angle (0.001 degrees) angular unconformity associated with tilting and truncation of the underlying sequences. Oxfordian sequences were probably never deposited in Interior Oman because of a lack of accommodation space and prolonged subaerial exposure. It is here proposed that the Upper/Middle Jurassic angular unconformity in Interior Oman was planed-off by subaerial carbonate dissolution during a steady, tectonically-driven uplift of the whole eastern Arabian shelf edge. The proposed geological model has several implications for the petroleum systems of Interior Oman. The geometric model predicts the distribution of the sedimentary facies, including source rocks, clastic and carbonate reservoirs, and seal facies. The occurrence of isolated Upper Mafraq-producing reservoir sands (i.e. Sayh Rawl field) are believed to be restricted to central and eastern Interior Oman. There are two other reservoir/seal combinations, both related to the Upper/Middle Jurassic unconformity: (1) truncation traps of the Dhruma-Tuwaiq below the unconformity (i.e. Hadriya and Uwainat reservoirs); (2) updip pinch-out trap of the Hanifa above the unconformity. Finally, it is believed that the early Late Jurassic general uplift and truncation of eastern Oman may have caused local remobilisation, updip migration, and loss to the surface of oil in reservoirs, initially generated from the prolific Al Huqf source rocks of Late Precambrian-Early Cambrian age.

Constraining sequence stratigraphy in north Australian basins: SHRIMP U–Pb zircon geochronology between Mt Isa and McArthur River

Fifty‐five new SHRIMP U–Pb zircon ages from samples of northern Australian ‘basement’ and its overlying Proterozoic successions are used to refine and, in places, significantly change previous lithostratigraphic correlations. In conjunction with sequence‐stratigraphic studies, the 1800–1580 Ma rock record between Mt Isa and the Roper River is now classified into three superbasin phases—the Leichhardt, Calvert and Isa. These three major depositional episodes are separated by ∼20 million years gaps. The Isa Superbasin can be further subdivided into seven supersequences each 10–15 million years in duration. Gaps in the geological record between these supersequences are variable; they approach several million years in basin‐margin positions, but are much smaller in the depocentres. Arguments based on field setting, petrography, zircon morphology, and U–Pb systematics are used to interpret these U–Pb zircon ages and in most cases to demonstrate that the ages obtained are depositional. In some instances, zircon crystals are reworked and give maximum depositional ages. These give useful provenance information as they fingerprint the source(s) of basin fill. Six new ‘Barramundi’ basement ages (around 1850 Ma) were obtained from crystalline units in the Murphy Inlier (Nicholson Granite and Cliffdale Volcanics), the Urapunga Tectonic Ridge (‘Mt Reid Volcanics’ and ‘Urapunga Granite’), and the central McArthur Basin (Scrutton Volcanics). New ages were also obtained from units assigned to the Calvert Superbasin (ca 1740–1690 Ma). SHRIMP results show that the Wollogorang Formation is not one continuous unit, but two different sequences, one deposited around 1730 Ma and a younger unit deposited around 1722 Ma. Further documentation is given of a regional 1725 Ma felsic event adjacent to the Murphy Inlier (Peters Creek Volcanics and Packsaddle Microgranite) and in the Carrara Range. A younger ca 1710 Ma felsic event is indicated in the southwestern McArthur Basin (Tanumbirini Rhyolite and overlying Nyanantu Formation). Four of the seven supersequences in the Isa Superbasin (ca 1670–1580 Ma) are reasonably well‐constrained by the new SHRIMP results: the Gun Supersequence (ca 1670–1655 Ma) by Paradise Creek Formation, Moondarra Siltstone, Breakaway Shale and Urquhart Shale ages grouped between 1668 and 1652 Ma; the Loretta Supersequence (ca 1655–1645 Ma) by results from the Lady Loretta Formation, Walford Dolomite, the upper part of the Mallapunyah Formation and the Tatoola Sandstone between ca 1653 and 1647 Ma; the River Supersequence (ca 1645–1630 Ma) by ages from the Teena Dolomite, Mt Les and Riversleigh Siltstones, and Barney Creek, Lynott, St Vidgeon and Nagi Formations clustering around 1640 Ma; and the Term Supersequence (ca 1630–1615 Ma) by ages from the Stretton Sandstone, lower Doomadgee Formation and lower part of the Lawn Hill Formation, mostly around 1630–1620 Ma. The next two younger supersequences are less well‐constrained geochronologically, but comprise the Lawn Supersequence (ca 1615–1600 Ma) with ages from the lower Balbirini Dolomite, and lower Doomadgee, Amos and middle Lawn Hill Formations, clustered around 1615–1610 Ma; and the Wide Supersequence (ca 1600–1585 Ma) with only two ages around 1590 Ma, one from the upper Balbirini Dolomite and the other from the upper Lawn Hill Formation. The Doom Supersequence (<1585 Ma) at the top of the Isa Superbasin is essentially unconstrained. The integration of high‐precision SHRIMP dating from continuously analysed stratigraphic sections, within a sequence stratigraphic context, provides an enhanced chronostratigraphic framework leading to more reliable interpretations of basin architecture and evolution.

Abstract: Gulf of Mexico Basin Depositional Synthesis: Neogene Sequences, Depositional Systems, and Paleogeographic Evolution

ABSTRACT An industry-sponsored synthesis of the Cenozoic depositional history of the Gulf of Mexico basin has integrated well data from the onshore, shelf and upper slope areas with Feng's 1995 seismic stratigraphic interpretation of the deep basin. The first phase of the project identified 10 Neogene genetic sequences that record major depositional episodes of the Gulf basin. For each sequence, digitally recorded data include thickness, lithofacies, depositional systems, stratigraphic architecture, local depocenters and paleo-geographic features such as paleo-shelf edge positions and submarine canyons. These data are stored in GIS format (ARC/INFOTM). Results of this synthesis are summarized in a series of interpretative maps showing the depositional system paleogeography for each of 10 sequences throughout the Gulf basin: Lower Miocene 1 (24-18.2 Ma); Lower Miocene 2 (18.2-16 Ma); Middle Miocene (16-12.8 Ma); Upper Miocene (12.8-6.2 Ma); Miocene-Pliocene (Bul. 1) (6.2-4.9 Ma); Pliocene (Glob. alt.) (4.9-3.1 Ma); Pliocene (Lent. 1) (3.1-2.3 Ma); Pliocene (Ang. B) (2.3-1.6 Ma); Pleistocene (Trim. A) (1.6-.55 Ma); and Pleistocene (Sang.) (.55-.27 Ma). The maps are supplemented by a series of regional reference margin well and deep basin seismic cross sections.

Introduction to the petrology and infrared spectra of Shanxi coals, People's Republic of China

Twelve samples of coal from the three major depositional episodes which occurred on the North China Block during the Carboniferous, Permian and Jurassic Periods have been investigated. The samples varied in rank from high volatile bituminous coal to anthracite and, although the coals from the three sequences were formed from different plants under different climates, plots of random vitrinite reflectance against elemental H C ratios were similar to those previously reported from coals deposited in Euramerica and Gondwanaland. None of the samples which have been examined contained significant quantities of liptinite; the vitrinite consisted mainly of desmocollinite. Shanxi coals resemble petrologically those from Gondwanaland. Diffuse reflectance, Fourier transform infrared (FT-i.r.) spectra of twelve samples and of their residues after pyridine extraction are presented and the chemical structures of these vitrinite-inertinite coals are shown to be very similar to those of better known bituminous coals. Increasing maturity (rank) of the coals can be attributed to increase in the geothermal temperature to which they have been subjected and is accompanied by the expected increase in aromaticity and also by an increase in the protonation of the aromatic structures. The spectra suggest that vitrinite-inertinite rich coals from many different depositional settings have similar patterns of substitution around their aromatic rings, the similarity increasing with rank. Thus the chemistry of vitrinite-rich coals appears to be controlled by the possession of common hydroaromatic structures.

The Late Pleistocene geology of the Labrador Shelf

The Quaternary stratigraphic sequence on the Labrador Shelf records evidence of several glacial cycles and major climatic/ environmental changes. This summary describes the major depositional episodes preserved on the Labrador Shelf and assigns ages and depositional interpretations to the last deglacial sequence. For a more detailed description of the seismic stratigraphy and sample analysis used to interpret this summary, the reader is referred to Josenhans et al. (1986).

Depositional Supersequences Among Cenozoic Strata of U.S. Atlantic Continental Margin: ABSTRACT

Integration of outcrop, borehole, and seismic reflection stratigraphy reveals the broad-scale depositional framework of Cenozoic strata of the U.S. Atlantic margin. The principal feature of the framework is a series of depositional supersequences bounded by interregional erosional unconformities that can be traced from coastal plain outcrops to the continental rise. The Atlantic continental margin is divided into five depositional areas: the Salisbury, Albemarle, and Charleston embayments; a composite of the entire Atlantic coastal plain; and a composite of the Atlantic shelf and slope. The major depositional episodes are early and middle Eocene in all depositional areas, late Eocene in all areas except the Albemarle embayment, early Oligocene on the composite shelf and s ope, late Oligocene in all depositional areas but minor in the Salisbury embayment, and early, middle, late Miocene and Pliocene in all areas. Paleoenvironmental and paleo-oceanographic analyses of the sediments and associated fossil assemblages indicate that alternation of major depositional and erosional episodes is controlled chiefly by the relative position of sea level. Thus, the supersequences correlate with the supercycles of the Vail sea level model. The bounding unconformities, in turn, correlate with the major global unconformities of the Vail model. Local variations among such things as terrigenous sediment supply, subsidence rates, and the position of major geostrophic currents, however, may accentuate or diminish the sea level effects. End_of_Article - Last_Page 1445------------

Quaternary Depositional Episodes, Western Spitsbergen, Norway: Aminostratigraphy and Glacial History

Quaternary events interpreted from deposits along the central west coast of Spitsbergen are subdivided and correlated by amino acid enantiomeric ratios in enclosed molluscan fossils. Deposition is characterized by relatively short episodes of rapid sedimentation associated with glacial activity and high relative sea level. Depositional episodes are separated by relatively longer periods of subaerial exposure and pedogenesis. At least five major depositional episodes, each terminated by an emergence cycle, are identified by characteristic isoleucine epimerization ratios (aIle/Ile) in Mya truncata and Hiatella arctica. Absolute age limitations are derived by calibrating the aIle/Ile ratios against radiometrically dated samples and diagnostic faunal communities. Glacial activity and sea-level change are inferred from the facies relationships within each depositional episode. The oldest recognized unit, episode H, is characterized by aIle/Ile Total ratios of 0.15 ? 0.02. At least three emergence cycles and two distinct glacial events are recorded within deposits of this episode. The absolute age is estimated to be between 300 and 1000 ka. The next younger episode, D, is represented at only one site, although reworked shells with the same ratios were also found. Total aIle/Ile ratios are ca. 0.08 and the age is estimated to be between 170 and 480 ka. Deposits of the three youngest depositional episodes, C, B, and A, are widespread throughout the study area, and are characterized by Total aIle/Ile ratios of 0.051 ?0.006, 0.033 ?+0.0022, and 0.019 ?0.001, respectively. Episode C deposits represent the last major regional glaciation of the area, an event estimated to have occurred between 90 and 260 ka ago. Episode B deposits are associated with a restricted regional glaciation estimated to have occurred between 40 and 130 ka ago. The associated fauna indicates marine conditions as warm or warmer than present. Episode A deposits are related to a latest Weichselian/earliest Holocene marine transgression/regression cycle during a period of restricted local glacier expansion, ca. 9 to 12 ka ago. These findings place constraints on the timing and extent of Spitsbergen/Barents Sea glacial events. The 18 ka BP open-ocean 6180 minimum is not associated with extensive glacierization of the Forlandsund region of western Spitsbergen, nor did outlet glaciers of the postulated Barents Sea ice sheet reach the study area in the last 40 ka or more.

Late Tertiary and Quaternary Depositional Systems in Subsurface of Central Texas Coastal Plain: ABSTRACT

Late Miocene, Pliocene, and Pleistocene deposits in the subsurface of the central Texas coastal plain were subdivided into six operational units equivalent to the surface-defined Fleming, Goliad, Willis, Lissie, and Beaumont Formations. These sedimentary units constitute the last major depositional episodes in the northwestern Gulf Coast basin. Late Miocene deposition is represented by transgressive shelf and shallow-marine shales overlain by progradational clastics of the upper part of the lower Fleming, upper Fleming, and lower Goliad-Willis units. A minor Pliocene transgressive event is represented by downdip, marine embayment facies of the upper Goliad-Willis unit. Finally, Pleistocene highstand fluviodeltaic progradation (Lissie and Beaumont units) terminated pre-Hol cene sedimentation. Interpretation of sediment distribution, established by constructing a series of net and percentage sand-maps for each unit, permits delineation of the following main depositional systems: fluvial braided-meander belt and flood basin; fluviodeltaic; lagoon; large marine embayments; small bayhead deltas; thick wave-dominated deltas; strand plain; and thick stacked coastal barriers. Western fluviodeltaic systems were consistently less active than the eastern ones, which deposited greater volumes of sand. Inherited, subtle structural influence of the deeper seated San Marcos arch had some effect on sediment distribution and paleogradients. Shallow extensions of the deeper Vicksburg, Frio, and Miocene fault systems display respectively decreasing (from 400 ft or 122 m) displacements in the section studied. Faults clearly were a central factor in the distribution of fluvial, deltaic, and strike-oriented coastal sands. Most sands in the updip parts of the operational units contain fresh water, whereas those of downdip areas contain predominantly brackish to saline waters. The area with greatest reservoir potential for fresh water includes Victoria, Jackson, Wharton, and Colorado Counties. Possible use of sealed, thick coastal sands in the lower Fleming unit for the disposal of industrial and municipal liquid-waste is recommended. End_of_Article - Last_Page 1689------------