Reconstruction Of Depositional Environments Research Articles

Early Mesozoic Detrital and Evaporitic Syn-Rift Series of Mohammedia-Benslimane-ElGara-Berrechid Basin (Meseta, Morocco): Sedimentary and Palaeoenvironmental Evolution and Comparison with Neighboring Basins

The Triassic basin of Mohammedia-Benslimane-ElGara-Berrechid contains a sedimentary series consisting of a sandy-conglomeratic formation (Formation A) at the base, surmounted by an argillaceous-saliferous formation (Formation B), the whole being capped by Late Triassic-Early Jurassic basalts. Formation (A) is formed by conglomeratic facies with alternations of sandstone in the middle and top of the formation. Formation (B) is subdivided into two members: (1) mudstone-siltstone member formed by alternations of mudstone and siltstone facies; and an argillaceous-saliferous member (2) composed by mudstone and evaporite facies. Sedimentary analysis carried out on more than ten outcrops and on four cores sample that have identified fifteen types of facies and seven architectural element and facies associations. It leads to the reconstruction of depositional environments during the Upper Triassic in the basin. Also, this work allowed a comparison of the sedimentary series of this basin with those of the other basins of the Moroccan Atlantic margin on one side and the North-east American margin on the other side.

Sandy contourite drift in the late Miocene Rifian Corridor (Morocco): Reconstruction of depositional environments in a foreland-basin seaway

The Rifian Corridor was a seaway between the Atlantic Ocean and the Mediterranean Sea during the late Miocene. The seaway progressively closed, leading to the Messinian Salinity Crisis in the Mediterranean Sea. Despite the key palaeogeographic importance of the Rifian Corridor, patterns of sediment transport within the seaway have not been thoroughly studied.In this study, we investigated the upper Miocene sedimentation and bottom current pathways in the South Rifian Corridor. The planktic and benthic foraminifera of the upper Tortonian and lower Messinian successions allow us to constrain the age and palaeo-environment of deposition. Encased in silty marls deposited at 150–300m depth, there are (i) 5 to 50m thick, mainly clastic sandstone bodies with unidirectional cross-bedding; and (ii) 50cm thick, mainly clastic, tabular sandstone beds with bioturbation, mottled silt, lack of clear base or top, and bi-gradational sequences. Furthermore, seismic facies representing elongated mounded drifts and associated moat are present at the western mouth of the seaway.We interpret these facies as contourites: the products of a westward sedimentary drift in the South Rifian Corridor. The contourites are found only on the northern margin of the seaway, thus suggesting a geostrophic current flowing westward along slope and then northward. This geostrophic current may have been modulated by tides. By comparing these fossil examples with the modern Gulf of Cadiz, we interpret these current-dominated deposits as evidence of late Miocene Mediterranean overflow into the Atlantic Ocean, through the Rifian Corridor. This overflow may have affected late Miocene ocean circulation and climate, and the overflow deposits may represent one of the first examples of mainly clastic contourites exposed on land.

Depositional environment reconstruction of the Maragheh Formation, East Azarbaijan, Northwestern Iran

The Maragheh Formation is an important deposit, which yields the savanna-type large mammal assemblage known as the “Pikermian Fauna.” Our high-resolution facies analysis of the interval between the Lower Pumice and the White Tuff demonstrated that debris-flow deposits and paleosols are dominant in the studied sequence. Fluvial channel-fill and small pond facies are the subordinate components in this interval. Most of the channel-fill deposits are interpreted as having been accumulated from ephemeral streams. The wide distribution of the Middle Pumice, the fact that it contains grains with older ages than those of the Lower Pumice, and the presence of “traction carpet” deposits allow the interpretation of the pumice interval as having been deposited from a hyperconcentrated flow probably caused by crater-lake destruction around the peak of Mt. Sahand, which supplied older rocks to the flow. The internal architecture of fluvial channel-fill deposits and the structures of paleosols (rhizoliths, cracks and slickensides: probable Vertic Inceptisols) imply a seasonal climate during deposition of the studied interval. This is consistent with previous environmental reconstructions based on mammal fossils (woodland-dominated savannah) as well as the results of phytolith analysis and δ18O data obtained from northern Iran. However, flood deposits covering the paleosols, showing sheet-like geometry, may not have been affected by large trees on the flood plain: the extent of woodland around the study site appears to have been limited.

Paleogene and Neogene reference sections of Eastern Kamchatka

The available lithological and paleontological data are used to characterize key Cenozoic marine sections in two lithological–facies zones (Kamchatskii Mys Peninsula and southern Valaginskii Range) of the poorly investigated Eastern Kamchatka lithotectonic zone with their correlation and reconstruction of depositional environments at the ocean–continent transition during the Paleogene and Neogene.

The influence of subaquatic springs in lacustrine sedimentation: Origin and paleoenvironmental significance of homogenites in karstic Lake Banyoles (NE Spain)

Banyoles (42°08′N, 2°45′E) is the largest and deepest lake of karstic–tectonic origin in the Iberian Peninsula. The lake comprises two basins and six sub-circularly shaped sub-basins fed by subaquatic springs. Periods of intense groundwater inflow in the deepest sub-basins lead to the fluidization and re-suspension of previously deposited sediments and subsequent settling forming homogenite deposits on the southern basin intermediate platforms. The multiproxy analysis of sediment cores combined with high resolution seismic stratigraphy (3.5kHz pinger and multi-frequency Chirp surveys) allows a precise reconstruction of depositional environments and related hydrological variability and groundwater inflow during the last ca. 7.6cal kyr BP. According to the age model based on 137Cs, 210Pb and AMS 14C dating, homogenite deposition occurred between 7.2 and 5.5cal kyr BP, stopped during the middle Holocene (5.5–2.8cal kyr BP) and greatly increased during the last two millennia with a total of 17 homogenite layers individually up to 75cm-thick. The onset of this unique sedimentation mode at ca. 3cal kyr BP coincides with an increase in lake level, evidenced by the onlapping of fine-grained, distal sediments over coarser massive, carbonate-rich, littoral deposits. A detailed, multidisciplinary study of the homogenites (sedimentology, physical properties, high-resolution elemental geochemistry, mineral composition, grain-size, organic matter content and SEM) combined with seismic stratigraphy demonstrates that the fluidization events triggering the formation of the homogenites were caused by higher and more intense local groundwater inflow, related to increased rainfall during the Late Holocene and likely intensified by land use changes during the last millennium.

Calcareous benthonic foraminifera across the Cretaceous/Paleocene transition of Gebel Um El-Ghanayem, Kharga Oasis, Egypt

The studies of benthic calcareous foraminifera of the Maastrichtian–early Paleocene Dakhla Formation in Gebel Um El-Ghanayem (Western Desert, Egypt), improve reconstruction of depositional environments of these successions. In total, 68 taxa of benthic foraminifera were identified in the studied succession.The late Maastrichtian assemblages (Zone CF3) are dominated by calcareous foraminifera with tapered tests, this tapered taxon Loxostomum applinae, Lox. tegulatum various dentalinid taxa, and Buliminella cushmani dominate in CF3 Biozone. We thus interpret these faunas as being dominated by infaunal morphogroups, suggesting a moderately eutrophic environment.Danian assemblages are characterized by abundant epifaunal trochospiral species, such as Cibicidoides abudurbensis, Cibicidoides farafraensis, and Gyroidinoides girardanus. The infaunal morphogroups make up 25–47% of fauna in the Danian, in contrast to 62–76% in the Upper Maastrichtian. This dominance of the Danian benthic foraminiferal assemblages by epifaunal or mixed epifaunal/infaunal morphogroups suggests that the food supply to the benthos was less abundant than in the latest Cretaceous.The Cretaceous/Paleocene boundary (K/Pg) is within the upper unit of the Lower Kharga Member and marked by a hiatus in at least the top of CF3 Zone of the Upper Maastrichtian to the Lower Paleocene (base Plc Zone).

Reconstruction of depositional environment of a tropical estuary and response of δ13Corg and TOC/TN signatures to changing environmental conditions

The distribution of sediment components, organic carbon, aluminium normalized metals, bulk sedimentary organic stable carbon isotope ratios i.e. δ13Corg and elemental TOC/TN ratios were studied within the sediment of Vaitarna estuary. Deposition of coarser sediment was favored under higher energy conditions prevailing in the past. Prevalence of relatively calm environment in recent years favored deposition of finer particles. Changes in depositional environment of Vaitarna estuary were very well reflected in distribution pattern of metals, δ13Corg and TOC/TN ratios. The narrow range (−22.9‰ to −22.0‰) and absence of distinct trend for δ13Corg in core collected towards the mouth (core S4) was attributed to prevalence of higher hydrodynamic energy conditions at this location. While protected environment of core S9 indicated a gradual change in the proportion of organic matter input to the estuary over a period of time. δ13Corg values in this core increased from −28.4‰ at the bottom to −20.7‰ at the surface and showed a corresponding decrease in TOC/TN ratio. All these changes will significantly alter the biogeochemical processes within the Vaitarna estuary and adjoining coastal waters.

North Andean forest dynamics mimics Greenland ice cores: changing vegetation, lake-levels, geochemistry, and sediment flux at ∼50-yr resolution during the last 284 ka

Banyoles (42°08′N, 2°45′E) is the largest and deepest lake of karstic–tectonic origin in the Iberian Peninsula. The lake comprises two basins and six sub-circularly shaped sub-basins fed by subaquatic springs. Periods of intense groundwater inflow in the deepest sub-basins lead to the fluidization and re-suspension of previously deposited sediments and subsequent settling forming homogenite deposits on the southern basin intermediate platforms. The multiproxy analysis of sediment cores combined with high resolution seismic stratigraphy (3.5 kHz pinger and multi-frequency Chirp surveys) allows a precise reconstruction of depositional environments and related hydrological variability and groundwater inflow during the last ca. 7.6 cal kyr BP. According to the age model based on 137Cs, 210Pb and AMS 14C dating, homogenite deposition occurred between 7.2 and 5.5 cal kyr BP, stopped during the middle Holocene (5.5–2.8 cal kyr BP) and greatly increased during the last two millennia with a total of 17 homogenite layers individually up to 75 cm-thick. The onset of this unique sedimentation mode at ca. 3 cal kyr BP coincides with an increase in lake level, evidenced by the onlapping of fine-grained, distal sediments over coarser massive, carbonate-rich, littoral deposits. A detailed, multidisciplinary study of the homogenites (sedimentology, physical properties, high-resolution elemental geochemistry, mineral composition, grain-size, organic matter content and SEM) combined with seismic stratigraphy demonstrates that the fluidization events triggering the formation of the homogenites were caused by higher and more intense local groundwater inflow, related to increased rainfall during the Late Holocene and likely intensified by land use changes during the last millennium.

Modes of fossil barnacle occurrences reflecting transgression in the Miocene Natori Group of the Moniwa-Goishi area, northeast Japan

Modes of occurrence of fossil barnacle reflecting transgression are discovered from the basal part of the Miocene Natori Group in the Moniwa-Goishi area. The group exhibits the transgressive sequence from subaerial lavas and volcaniclastics to shoreface-shelf deposits in ascending order. Fossil barnacles show autochthonous (in situ individuals on gravel surface) to allochthonous (disarticulated fragments) occurrences. Autochthonous barnacle species, Balanus bisulcatus and Concavus sendaicus are found from bay deposits of the early transgressive stage, and the overlying ravinement deposits formed during marine flooding, respectively. Discoveries of autochthonous barnacles make possible reconstruction of depositional environments and paleoecolgy of extinct barnacles.

Maastrichtian to Paleocene depositional environment of the Dakhla Formation, Western Desert, Egypt: sedimentology, mineralogy, and integrated micro- and macrofossil biostratigraphies

Integrated sedimentology, mineralogy, geochemistry, and microfossil and macrofossil biostratigraphies of the Maastrichtian–early Paleocene Dakhla Formation of the Western Desert, Egypt, provide improved age resolution, information on the cyclic nature of sediment deposition, and the reconstruction of depositional environments. Age control based on integrated biostratigraphies of planktic foraminifera, calcareous nannofossils and macrofossils yields the following ages for stratigraphic and lithologic sequences. The contact between the Duwi and Dakhla formations marks the Campanian/Maastrichtian boundary (zone CF8a/b boundary) and is dated at about 71Ma. The age of the Dakhla Formation is estimated to span from 71Ma at the base to about 63Ma at the top (zones CF8a–Plc). The Cretaceous/Tertiary (K/T) boundary is within the upper unit of the Kharga Shale Member and marked by a hiatus that spans from 64.5Ma in the lower Paleocene (base Plc) to at least 65.5Ma (base CF2, base M. prinsii zones) in the upper Maastrichtian at Gebel Gifata, the type locality of the Dakhla Formation. As a result, the Bir Abu Minqar horizon, deposited between about 64.2 and 64.5Ma (Plc(l) zone), directly overlies the K/T boundary hiatus. Major hiatuses also span the late Maastrichtian–early Paleocene in sections to the northwest (c.61.2–65.5Ma at North El Qasr, c.61.2–69Ma at Bir Abu Minqar and c.61.2–65.5Ma at Farafra), and reflect increased tectonic activity.During the Maastrichtian–early Paleocene a shallow sea covered the Western Desert of Egypt and the clastic sediment source was derived primarily from tectonic activity of the Gilf El Kebir spur to the southwest of Dakhla and the Bahariya arch. Uplift in the region resulted in major hiatuses in the late Maastrichtian–early Paleocene with increased erosion to the southwest. The area was located near the palaeoequator and experienced warm, wet, tropical to subtropical conditions characterized by low seasonality contrasts and predominantly chemical weathering (high kaolinite and smectite). A change towards perennially more humid conditions with enhanced runoff (increased kaolinite) occurred towards the end of the Maastrichtian and in the early Paleocene with shallow seas fringed by Nypa palm mangroves. Sediment deposition was predominantly cyclic, consisting of alternating sandstone/shale cycles with unfossiliferous shales deposited during sea-level highstands in inner neritic to lagoonal environments characterized by euryhaline, dysaerobic or low oxygen conditions. Fossiliferous calcareous sandstone layers were deposited in well-oxygenated shallow waters during sea-level lowstand periods.

Architectural element analysis applied to glacial deposits: Internal geometry of a late Pleistocene till sheet, Ontario, Canada

Research Article| January 01, 2000 Architectural element analysis applied to glacial deposits: Internal geometry of a late Pleistocene till sheet, Ontario, Canada Joseph I. Boyce; Joseph I. Boyce 1School of Geography and Geology, McMaster University, Hamilton, Ontario L8S 4K1, Canada Search for other works by this author on: GSW Google Scholar Nicholas Eyles Nicholas Eyles 2Glaciated Basin Research Group, University of Toronto, Scarborough, Ontario M1C 1A4, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Joseph I. Boyce 1School of Geography and Geology, McMaster University, Hamilton, Ontario L8S 4K1, Canada Nicholas Eyles 2Glaciated Basin Research Group, University of Toronto, Scarborough, Ontario M1C 1A4, Canada Publisher: Geological Society of America Received: 23 Apr 1998 Revision Received: 20 Dec 1998 Accepted: 11 Jan 1999 First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2000) 112 (1): 98–118. https://doi.org/10.1130/0016-7606(2000)112<98:AEAATG>2.0.CO;2 Article history Received: 23 Apr 1998 Revision Received: 20 Dec 1998 Accepted: 11 Jan 1999 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Joseph I. Boyce, Nicholas Eyles; Architectural element analysis applied to glacial deposits: Internal geometry of a late Pleistocene till sheet, Ontario, Canada. GSA Bulletin 2000;; 112 (1): 98–118. doi: https://doi.org/10.1130/0016-7606(2000)112<98:AEAATG>2.0.CO;2 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 SocietyGSA Bulletin Search Advanced Search Abstract Deposits left by continental ice sheets are characterized by sedimentological complexity and stratigraphic heterogeneity, but stratigraphic descriptions of such deposits, and resulting “first-generation” facies models, are still based primarily on one- or two-dimensional borehole or outcrop data. Reconstruction of depositional environments, hydrogeological investigations of Pleistocene glacial deposits, and hydrocarbon exploration in pre-Pleistocene glaciated basin fills require a more detailed understanding of the form and heterogeneity of lithofacies sequences in three dimensions.Architectural element analysis is used widely by sedimentologists for categorizing internal stratigraphic heterogeneity in sandstones, particularly those of fluvial origin. This paper demonstrates the first application of architectural element analysis to glacial deposits such as tills. Outcrop, borehole, and a broad range of subsurface geophysical data were collected from a thick (60 m) till sheet present across an 80 km2 study area near Toronto, Canada. The till sheet is not homogeneous, but is composed of three distinct architectural elements and associated lithofacies, viz, diamict elements, interbeds of subglaciofluvial sediments, and glaciotectonically deformed zones. Application of architectural element analysis to these subglacial strata provides insights into the origin of drumlin bedforms and subglacial processes below the Laurentide Ice Sheet and creates a framework for understanding ground-water and contaminant movement in underlying aquifers. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Hydrodynamics of Bivalve Shell Entrainment and Transport

ABSTRACT The relationship between the hydrodynamic properties of disarticulated bivalve shells, and the entrainment and transport of these shells by water currents, was studied by means of flow-visualization experiments and measurements of surface pressure around the shell bodies. The study was performed on eleven bivalve shells, simulated as resting convex-up on the bottom, with their beaks pointing into the flow. The results indicate that bivalve shells respond to hydrodynamic processes in a systematic and predictive manner. Drag and lift were best represented by an exponential relationship between the sum of the projected plan and frontal area of the shells. This relationship supports the argument that greater forces are generated for large, convex and elongated shells. In addition, lift-to drag ratio increased with increasing thickness index (height/length). Roughness elements on shells, such as ribs, produce a drag-reduction transition at high Reynolds numbers, as with spheres, but this transition occurs at lower Reynolds numbers than for spheres. Asymmetry in shells contributes to the development of drag and lift. Large lateral forces were found to be directly related to asymmetry in plan shape. Force-to-shape relationships can be applied to any given shell to determine its transportability and assess the influence of hydrodynamic processes on fossil assemblages. Force resolution was used to calculate entrainment velocities for shells, which were compared with entrainment velocities measured in a recirculating flume. Calculated velocities of 0.2-0.8 m/s were in good agreement with measured velocities. Calculations of entrainment velocity can be used to estimate paleocurrent velocity in the reconstruction of depositional environments.

Extended 3β-alkyl steranes and 3-alkyl triaromatic steroids in crude oils and rock extracts

In oils and Precambrian- to Miocene-age source rocks from varying depositional environments, we have conclusively identified several novel 3-alkyl sterane and triaromatic steroid series, including (1) 3β-n-pentyl steranes, (2) 3β-isopentyl steranes, (3) 3β-n-hexyl steranes, (4) 3β-n-heptyl steranes, (5) 3,4-dimethyl steranes, (6) 3β-butyl,4-methyl steranes, (7) triaromatic 3-n-pentyl steroids, and (8) triaromatic 3-isopentyl steroids. We have also tentatively identified additional homologs with 3-alkyl substituents as large as C11. The relative abundances of these compounds vary substantially between samples, as indicated by (1) the ratio of 3β-n-pentyl steranes to 3β-isopentyl steranes and (2) the ratio of 3-n-pentyl triaromatic steroids to 3-isopentyl triaromatic steroids. These data suggest possible utility for these parameters as tools for oil-source rock correlations and reconstruction of depositional environments. Although no 3-alkyl steroid natural products are currently known, several lines of evidence suggest that 3β-alkyl steroids result from bacterial side-chain additions to diagenetic Δ2-sterenes.

Sulfate Platform-Basin Transition of the Lower Werra Anhydrite (Zechstein, Upper Permian), Western Poland: Facies and Petrography

ABSTRACT The peripheral evaporite platform of the Zechstein (Upper Permian) Werra Anhydrite of western Poland comprises a series of shoals (with thick sulfate deposits) and lows (with thin sulfate and thick halite deposits). Three cores were selected to examine facies variations from a sulfate platform, slope, and basin. The basal unit of the Lower Werra Anhydrite is represented by different varieties of nodular, bedded-nodular, fluidal-nodular, and brecciated facies. Some nodular fabrics probably originated within sabkha and/or gypsumpond environments, and the deposit was then mechanically redistributed. The absence of nodular structures in units other than the lower one in two of the wells suggests that the origin of these structures cannot be related to deep burial. Other units of the Lower Werra Anhydrite represent subaqueous deposits. The facies in the middle and upper units have no recent analog in coastal salina environments. Turbidity currents displaced sulfates from the platform to the adjacent basin. The origin of lenticular-bedded anhydrite may be related to deformation by compaction or by ravitational instability and slumping, which may have initiated crystallization or recrystallization of anhydrite. The vertical sequence of facies shows a distinct deepening-upward trend starting from the top of the basal nodular units. Deposition in shallow (salina) environments dominated the early history of the Lower Werra Anhydrite basin, although it was probably preceded by transgressive sabkha deposits. Relatively thick nodular anhydrites probably formed by syndepositional diagenesis of probable salina deposits and minor sabkha sediments. Gypsum was the original sedimentary mineral; it was later transformed into anhydrite, during either early or late (burial) diagenesis. We show that in deeply buried evaporites not only some macroscopic primary textures remain but also even microscopic details, which allow reconstruction of depositional environments and primary mineralogy of ancient anhydrites.

Pyrite-organic matter relationships: Currant Bush Limestone, Georgina Basin, Australia

The relationship between the TOC (total organic carbon) and S (pyritic sulphur) contents of some organic-rich (TOC 3–14%) silty limestones and calcareous siltstones from the Cambrian Currant Bush Limestone of the Georgina Basin, Australia, is similar to that of euxinic sediments, but could also indicate deposition in a non-saline environment. However, such conditions are not easily reconciled with the shallow equatorial epeiric sea in which the formation is considered to have been deposited. The relationship can alternatively be explained by the relatively low Fe content of the sediment being the main limiting factor in pyrite generation, coupled with a slightly increased degree of pyritization with increased TOC. The relationship between the TOC and S contents of organic-poor (TOC 0.2–2.4%) samples from the same formation is similar to that of normal marine sediments, but the Fe content is probably also the major controlling factor on pyrite generation. These interpretations demonstrate that in using TOC and S contents to aid in reconstruction of depositional environments, it is important to check whether Fe content could have been the limiting factor in the formation of pyrite, even in sediments with only moderate carbonate contents.

Role of clastic sedimentology in the exploration and production of oil and gas in the North Sea

Summary The nature and quality of clastic reservoir rocks is the result of a complex interplay of tectonic setting, provenance, depositional environment and diagenesis. In particular, it is now well established that the reconstruction of depositional environments in clastic successions provides the optimum framework for describing and predicting reservoir development and reservoir quality distribution on both regional (exploration) and field (production) scales. This review, in the context of the 21st anniversary of the British Sedimentological Research Group (BSRG), expands the above-mentioned theme in relation to the development of depositional models of each of the major oil- and/or gas-bearing clastic reservoirs in the U.K. sector of the North Sea, which has been the subject of a surge in geological interest following the first gas discoveries approximately 21 years ago. Each major reservoir interval is used to illustrate specific themes, with emphasis placed on how the application of depositional models has contributed to a better understanding of the exploration for, and the production from, the following reservoirs: (i) Permian aeolian and fluvial sands (Rotliegendes Sandstones, southern North Sea), (ii) Jurassic fluvial, deltaic and marginal marine sands (Lower Jurassic Statfjord Formation and Middle Jurassic Brent Group, northern North Sea), (iii) Jurassic shallow marine and deeper marine (mass flow) sands and conglomerates (Upper Jurassic Fulmar/Piper-type sands and Brae/Magnus-type deposits; central and northern North Sea), and (iv) Tertiary deep marine, turbidite fan deposits (Palaeocene and Eocene sands, central and northern North Sea).

Recent Advances in Petroleum Geochemistry Applied to Hydrocarbon Exploration

This paper presents a critical review of recent advances in the study of generation, maturation, and migration of petroleum. The different source potentials for generating hydrocarbons depend on the type of kerogen. In addition to the classic types I, II, and III, a residual, oxidized organic matter is also observed, which has no potential for hydrocarbon generation. Coal is widely considered as a potential source for gas, but some coals may generate sizeable amounts of oil as well. The development of routine analytical tools, particularly pyrolysis, makes possible the preparation of geochemical logs that are the basis for screening samples and for interpreting other analyses. Among optical techniques, the major subjects of discussion concern the nature of amorphous organ c matter, which may encompass different chemical types of kerogen, and the applicability of vitrinite reflectance techniques to type I and II kerogens. Following important advances in identification of biological markers in sediments and crude oils, these markers are used for oil-source rock correlation, and proposed for reconstruction of depositional environment and subsequent thermal evolution. Migration of hydrocarbons is now better understood, and the importance of a hydrocarbon-phase migration is widely recognized. Overpressuring of pore fluids is mainly responsible for expulsion of petroleum. Geologic models of basin evolution (subsidence, compaction, thermal history) and geochemical models (hydrocarbon generation and migration) become progressively available. Their integration int a comprehensive model should be one of the major developments in future petroleum exploration.

Lithofacies types and vertical profile models; an alternative approach to the description and environmental interpretation of glacial diamict and diamictite sequences

ABSTRACT Increased knowledge of modern glacial depositional environments has resulted in rapidly evolving classifications of glacial tills. These are based to a large degree on theoretical considerations of likely depositional processes. The classifications are sophisticated and more advanced than the establishment of simple field criteria whereby individual till facies can be identified in Quaternary and Pre‐Quaternary successions. This situation is compounded in many Quaternary terrains by the continued description of ‘tills’ in terms of laboratory‐derived analytical data only, reflecting a traditional interest in stratigraphic correlation rather than reconstruction of depositional environment. Detailed sedimentological logging of lithofacies is rarely undertaken. There is thus considerable confusion as to what is being described or sampled when analytical data are presented for many Pleistocene ‘tills’. The same remarks apply to Pre‐Pleistocene ‘tillites’.A lithofacies code is presented here for the rapid description and visual appraisal of field sequences or drill cores containing unconsolidated diamicts or lithified diamictites; the term‘till’is not used as it has a strict genetic definition referring to direct aggregation and deposition by glacier ice. Use of a four part code, in conjunction with codes already published for fluvial sediments, allows fundamental field properties to be depicted independent of genetic terminology and provides a firm basis for subsequent environmental interpretation and analytical work. The value of this approach is illustrated by comparing a representative suite of vertical profiles of diamict assemblages deposited by modern grounded glaciers with a classic late Pleistocene glacigenic sequence at Scarborough Bluffs, Ontario.

5β-isomers of stanols and stanones as potential markers of sedimentary organic quality and depositional paleoenvironments

Organic molecules originating only from the in situ diagenesis of biogenic molecules are ideal geochemical fossils which may provide information essential for the characterization and reconstruction of depositional environments and subsequent chemical reactions during diagenesis. It is proposed herein that this is the case for the 5β-isomers of stanols and stanones produced during stenol hydrogenation in young aquatic sediments, if shown to be essentially free of any major anthropogenic pollution (particularly, sewage). In order to clarify the environmental factors controlling the production of the 5β-steroidal isomers from stenols in recent aquatic sediments, attempts were made to relate the occurrence of 5β-stanols to various environmental parameters. Positive correlations between elevated concentrations of 5β-stanols and the degree of autochthonous contribution to sedimentary organic matter were consistently found in various surface aquatic sediments from a wide variety of depositional environments and also in older sediments extending even to the late Pleistocene. According to this finding, it was concluded that the primary factor controlling the conversion of stenols to 5β-stanols through 5β-stanones in anaerobic aquatic sediments is probably the relative contribution of autochthonous organic matter suitable for microbial metabolism ( i.e. metabolizable organic matter) to the sediments. Consequently, it is proposed that the 5β-isomers of stanols and stanones, at least in immature aquatic (marine and non-marine) sediments, can serve as primary markers for defining the quality of sedimentary organic matter ( viz. the relative contribution of metabolizable organic materials to sedimentary organic matter) and as indicators for the types and rates of microbiological activities responsible for early diagenesis of organic matter in anaerobic sediments. It is also suggested that the combination of the 5β-steroidal isomers with organic source parameters will allow these compounds to assist in indicating oxic or anoxic depositional environments.

Controls on Pennsylvanian Hydrocarbon Accumulations in Mid-Continent: ABSTRACT

Approximately 8.8 billion bbl of oil and about 31.5 Tcf of gas have been found in Pennsylvanian reservoirs in the Mid-Continent as of January 1, 1978. Although these volumes of hydrocarbons were trapped in thousands of fields throughout the region, most of these resources were emplaced in a relatively few fields: about 6.4 billion bbl of oil has been found in 90 significant and giant oil fields, and 18.5 Tcf of gas has been discovered in 50 significant and giant gas fields. Our calculations of the total oil and gas accumulations in Pennsylvanian reservoirs were extrapolated from these data. Most oil and gas accumulations of Pennsylvanian age in the Mid-Continent were stratigraphically trapped in lenticular sandstone bodies; the environments in which most of the clastics were deposited range from fluvial to deltaic to shallow marine. Even though this region is now in a late mature stage of exploration and development, important stratigraphic accumulations of oil and gas remain to be found. These fields will be discovered through detailed subsurface analysis, the reconstruction of depositional environments, and the application of high-resolution seismic data to stratigraphic problems. End_of_Article - Last_Page 2119------------