Time-stratigraphic Units Research Articles

Miospore biozonation of cenomanian - santonian succession in Famo-1 well, Gongola Sub Basin, Upper Benue Trough, Nigeria

The Early Cretaceous succession penetrated by Famo-1 well in Gongola sub Basin, Upper Benue Trough, Nigeria were analyzed for its palynological content. This investigation produced significant Pollen and Spore and the assemblage reveals the dominance of angiosperm Pollen grain and pteridophytes spore. Gymnosperm pollen was scarce and less varied. The studied interval penetrated a sequence of sandstone, sandy shale and mudstone occurring at different intervals in the studied section of the well. Established on the stratigraphic distribution of the palynomorphs recovered from Famo-1 well, three informal assemblage palynozones covering the Early Cretaceous were identified. The zones are: Assemblage Zones I & II (Cenomanian); Assemblage Zone III (Turonian - Santonian). The different zones signifies a time stratigraphic unit comparable to the identified lithostratigraphic units penetrated by the well. The age (Cenomanian - Santonian) determinations are based on the known stratigraphic ranges of pollen and spores and their relative stratigraphic positions. Data from the studied well revealed that there is relatively higher frequency of the land derived pollen and spores compared to marine palynomorph abundance, which suggests a paralic condition of continental to shallow marine environment. The shallow marginal marine environment is further sustained by the presence of foraminifera test linings at some depths which are indicative of marine palaeoenvironment (inner neritic environment). Interpretations from the spore and pollen assemblages reveal sediment deposition took place in a wet to arid climate condition in marginal marine to nearshore environment.Keywords: Palynology, Gongola sub Basin, Cretaceous, Famo-1 Well

Chapter 5 Stratigraphic methodology for the geological mapping of volcanic areas: insights from the Aeolian archipelago (southern Italy)

Abstract A stratigraphic approach to geological fieldwork and mapping of the Aeolian archipelago is performed by primarily using unconformity-bounded units (UBUs) in addition to lithosomes and classical lithostratigraphic units, which are the basic mappable units. Lithosomes are largely used to identify the localization of eruptive vents or non-volcanic source areas through time. The UBUs display an encompassing framework of correlation on a scale from local (single islands) to regional (Aeolian archipelago), by placing particular emphasis on the stratigraphic role played by Late Quaternary marine terrace deposits and widespread tephra layers (e.g. Brown Tuffs). By establishing a direct relationship between UBUs and time-stratigraphic units, the volcanic successions are readable in terms of different and successive Eruptive Epochs (separated by prolonged non-volcanic periods), which are characterized by distinctive eruptive vents and eruption types. As such, the UBUs allow the reconstruction of the main steps of geological evolution of a volcano as the result of the interplay between volcanic activity of local and external provenance, sea-level fluctuations and volcano-tectonic or regional fault lineaments.

Global geological map of Venus

The surface area of Venus (∼460×106km2) is ∼90% of that of the Earth. Using Magellan radar image and altimetry data, supplemented by Venera-15/16 radar images, we compiled a global geologic map of Venus at a scale of 1:10M. We outline the history of geological mapping of the Earth and planets to illustrate the importance of utilizing the dual stratigraphic classification approach to geological mapping. Using this established approach, we identify 13 distinctive units on the surface of Venus and a series of structures and related features. We present the history and evolution of the definition and characterization of these units, explore and assess alternate methods and approaches that have been suggested, and trace the sequence of mapping from small areas to regional and global scales. We outline the specific defining nature and characteristics of these units, map their distribution, and assess their stratigraphic relationships. On the basis of these data, we then compare local and regional stratigraphic columns and compile a global stratigraphic column, defining rock-stratigraphic units, time-stratigraphic units, and geological time units. We use superposed craters, stratigraphic relationships and impact crater parabola degradation to assess the geologic time represented by the global stratigraphic column. Using the characteristics of these units, we interpret the geological processes that were responsible for their formation. On the basis of unit superposition and stratigraphic relationships, we interpret the sequence of events and processes recorded in the global stratigraphic column. The earliest part of the history of Venus (Pre-Fortunian) predates the observed surface geological features and units, although remnants may exist in the form of deformed rocks and minerals. We find that the observable geological history of Venus can be subdivided into three distinctive phases. The earlier phase (Fortunian Period, its lower stratigraphic boundary cannot be determined with the available data sets) involved intense deformation and building of regions of thicker crust (tessera). This was followed by the Guineverian Period. Distributed deformed plains, mountain belts, and regional interconnected groove belts characterize the first part and the vast majority of coronae began to form during this time. The second part of the Guineverian Period involved global emplacement of vast and mildly deformed plains of volcanic origin. A period of global wrinkle ridge formation largely followed the emplacement of these plains. The third phase (Atlian Period) involved the formation of prominent rift zones and fields of lava flows unmodified by wrinkle ridges that are often associated with large shield volcanoes and, in places, with earlier-formed coronae. Atlian volcanism may continue to the present. About 70% of the exposed surface of Venus was resurfaced during the Guineverian Period and only about 16% during the Atlian Period. Estimates of model absolute ages suggest that the Atlian Period was about twice as long as the Guineverian and, thus, characterized by significantly reduced rates of volcanism and tectonism. The three major phases of activity documented in the global stratigraphy and geological map, and their interpreted temporal relations, provide a basis for assessing the geodynamical processes operating earlier in Venus history that led to the preserved record.

6. British clay stratigraphy

There are three principal types of stratigraphy, each with its own terminology: geochronolog —units of geological time (e.g. period, epoch), with a parallel and exactly corresponding stratigraphy; chronostratigraphy —time-rock units or time-stratigraphic units represent stratified rock successions (e.g. system, series, etc.) assigned to geological time units; geochronometry —the measurement of absolute time in years as numerical ages (e.g. 345 Ma), principally by means of radiometric dating, but increasingly, in Phanerozoic rocks, by the numerical dating of Milankovitch cycles. This has led to some confusion. Current recommendations, led by the Geological Society (Zalasiewicz et al . 2004) are to adopt the geochronology scheme, which has been used in this chapter. Part of the geological timescale, covering the last 545 million years of earth's history (Phanerozoic—from the greek meaning ‘visible life’), is shown in Table 6.1 (Gradstein & Ogg 1996) with the youngest at the top. This shows the broadest sub-divisions of era and period (the Neogene and Palaeogene are often referred to as the Tertiary, though this name may fall into disuse). The names for the eras derive from the greek meaning ‘ancient’, ‘middle’, and ‘new’ life. Periods further sub-divide into early, middle, and late (or in some cases simply early and late), and thence into epochs and ages (refer to Appendix C). Time before the Phanerozoic is usually referred to as the Pre-Cambrian. Clays are found in three principal geological environments: as primary deposits in the sedimentary environment, as alteration products of other materials in the weathering and

Seismic frequency control on carbonate seismic stratigraphy: A case study of the Kingdom Abo sequence, west Texas

Conceptual models and real three-dimensional (3-D) seismic data show that in progradational carbonate platform margin and slope deposits of the Kingdom Abo reservoir of the Permian basin, west Texas, primary seismic reflection events do not necessarily follow clinoformal geologic-time surfaces. The seismic frequency content of the data controls the dip and architecture of seismic reflection events. High-frequency seismic data tend to follow thinner, time-bounded clinoform depositional elements (time-stratigraphic units), whereas low-frequency seismic data tend to image thicker, low-angle lithofacies units (time-transgressive units). In seismic data of moderate frequency, both clinoform units and flat lithofacies units are imaged, creating complex interference patterns that are difficult to interpret.Experiments with models and real data demonstrate that seismic data can be selectively filtered in the signal bandwidth to help distinguish time-stratigraphic units from lithostratigraphic units. Selective filtering alters the dominant frequency of the data to match a desired scale of geologic objects. If there are enough high-frequency components in the seismic data, true clinoform stratigraphy can be imaged even if the data are dominated by lower frequency components.Seismic modeling of outcrop of the Abo sequence in Apache Canyon, Sierra Diablo, west Texas, indicates that a dominant frequency of 100 Hz is needed to recover true clinoform stratigraphy using seismic data. The interpretation of available 3-D seismic data can only partially distinguish time-stratigraphy from lithostratigraphy because of the lack of frequency components greater than 70 Hz in the data. Application of this outcrop model in seismic modeling avoids interpretational pitfalls that can occur if the dominant frequency of the seismic data is not matched to the unit thicknesses that need to be resolved.

Geology and Origin of Waldo Lake, Oregon

ABSTRACT Volcanism has been active in the central Oregon Cascade Range for at least the last 35 Ma. During that period interactions between the Pacific, North American, and Juan de Fuca plates have produced variations in the composition and rate of volcanism, the landward migration and narrowing of the axis of volcanism, and discreet deformational events. The result of these effects is widespread unconformities that have enabled Cascade Range workers to identify five mappable time-stratigraphic units. Waldo Lake occupies a position in the High Cascades physiographic province. The surrounding rocks are primarily very young (less than 1–2 Ma) basaltic andesite lava flows covered with a thin veneer of glacial drift. Waldo Lake was carved by a northward-moving glacier and is dammed on its southeast side by a lateral moraine. The location of the lake is probably controlled by a large north-south trending down-to-the-east normal fault.

Offshore Atlas Project: Methodology and Results

The Offshore Atlas Project (OAP) grouped 4,325 Miocene and older and 5,622 Pliocene and Pleistocene productive sands in the Gulf of Mexico into 91 chronostratigraphic hydrocarbon plays to aid the oil and gas industry with regional hydrocarbon exploration and field development. OAP has produced a two-volume atlas series entitled Atlas of Northern Gulf of Mexico Gas and Oil Reservoirs. Volume 1 comprises Miocene and older reservoirs, while volume 2 comprises Pliocene and Pleistocene reservoirs. Chronozones (Reed et al., 1987) were used to define geologic ages in the Gulf of Mexico. A chronozone is a time-stratigraphic unit defined by a particular benthic foraminifera biostratigraphic zone. The 26 chronozones identified by Reed et al. (1987) were further grouped into 14 Cenozoic and 2 Mesozoic chronozones for OAP. A composite type log (CTL), which shows the chronostratigraphic relationship of all productive sands in a field, was constructed for each of the 876 proved federal fields in the Gulf of Mexico. Depositional facies (retrogradational, aggradational, progradational, and submarine fan)were next identified on each CTL. The four facies were identified primarily according to characteristic SP-curve shapes, paleoecozones, and sand content. The chronozones and depositional facies identified on each CTL were then correlated among fields across the Gulf of Mexico. All productive sands correlated to the same chronozone and depositional facies were then identified as a unique play. Both federal and state fields in the Gulf of Mexico contain original proved reserves (sum of cumulative production and remaining proved reserves) estimated at 12.481 Bbbl of oil and condensate and 156.466 Tcf of gas (40.322 Bboe [sum of liquids and energy equivalent gas]). Of this, 9.943 Bbbl of oil and condensate and 122.263 Tcf of gas (31.698 Bboe) have been produced. Miocene plays contain the most total original proved reserves with 41.9 %, followed by Pleistocene plays (36.2 %), Pliocene plays (18.6 %), Mesozoic plays (2.9 %), and Oligocene plays (0.4 %). Miocene plays have produced the largest amount of total hydrocarbons, as well, at 43.5 % followed by Pleistocene plays (36.5 %), Pliocene plays (19.1 %), Oligocene plays (0.5 %), and Mesozoic plays (0.4 %). Just over two-thirds of the Gulf of Mexico's total original proved reserves are contained in progradational facies (67.4 %),with the remainder comprising submarine-fan facies (18.5 %), aggradational facies (9.9 %), retrogradationalfacies (2.4 %), combination facies (1.7 %), and caprock and reef reservoirs (0.1 %). Total cumulative production from the different facies closely mimics the distribution of original proved reserves. Of the 91 plays, the lower Pleistocene progradational play (LPL P.1) contains the most original proved gas reserves (10.5 %) and has produced the most gas (11.4 %). However, the upper upper Miocene eastern progradational play (UM3 P.1B) contains the most original proved oil and condensate reserves (18.9 %) and has produced the most oil and condensate (21.4%). Several technical studies resulting from OAP have been published. Hunt and Burgess (1995) described the distribution of OAP plays deposited by the ancestral Mississippi River delta system in the north-central Gulf of Mexico over the past 24 million years. The lower Miocene plays are restricted to the westernmost portion of the Louisiana shelf. In the late middle Miocene, the depocenter migrated east of the present-day Mississippi River delta. During the late upper Miocene, the depocenter began migrating back to the west and prograded basinward, and it continued to do so throughout the Pliocene and Pleistocene. Lore and Batchelder (1995) discussed how OAP plays can be used to find exploration targets and assess undiscovered resources. As an exploration tool, OAP play maps can be used to identify conceptual submarine-fan plays downdip of established shallow water producing facies, and to identify wells where a known producing facies or chronozone has not yet been reached. As an assessment tool, the extensive data sets associated with each OAP play can be used to infer statistically the size of undiscovered resources in a play to determine if exploration in that play is economically justifiable. Lore et al. (1995) estimated the amount of undiscovered conventionally recoverable resources in the Gulf of Mexico, basing their assessment on previous work performed for OAP. Mean level estimates show that, by far, submarine-fan plays have the greatest potential for additional oil and gas in the Gulf of Mexico, with 75.1 % and 70.4 % of the total oil and gas resources, respectively. Mean level estimates for the 13 OAP Miocene, Pliocene, and Pleistocene chronozones show that upper Pleistocene plays have the most oil resource potential (24.3 %), while lower Pleistocene plays have the most gas resource potential (20.6 %).

Regional and global stratigraphy of Venus: a preliminary assessment and implications for the geological history of Venus

Photogeologic analysis of Magellan images for 36 widespread sites and several larger areas permits the definition and characterization of a sequence of mappable stratigraphic units and tectonic structures deforming them. Seven rock-stratigraphic units, three related time-stratigraphic units (Systems), and three geologic time subdivisions (Periods) are proposed to describe the vast majority of the areas under study. The first widespread geologic unit preserved on Venus is the highly deformed tessera terrain formed during the Fortunian Period, apparently the result of an event that destroyed the morphology of preexisting terrain and any superposed craters. Global image data reveal no evidence for extensive terrain dating from the pre-Fortunian time, comprising the first 80–90% of the history of Venus, although rocks dating from this period are almost certainly contained within the tessera. Immediately following the Fortunian an extensive period of plains volcanism began, the Guineverian Period, during which the majority of Venus was volcanically resurfaced. During the Guineverian early widespread plains of the Sigrun Group were deformed by extensive and closely-spaced graben systems. Continued widespread plains emplacement occurred and units of the Lavinia Group were deformed, some into extensive ridge belts, recording a change from distributed extensional deformation to often-focused compressional deformation. Plains of the Rusalka Group are the most widespread currently exposed, and are characterized by extensive development of wrinkle ridges of compressional origin. These Guineverian Period plains must have been emplaced and deformed over a relatively short period of time (probably less than about a hundred million years) because the vast majority of impact craters are superposed on the plains, and the crater retention age of this surface is of the order of 300–500 Ma. This extensive plains volcanism then gave way to materials of the Atla Group, local volcanic edifices and flow units with sources associated with coronae and rifts that were emplaced in the late Guineverian and Aurelian Periods. The Aurelian Period, defined by impact craters with dark parabolas, is interpreted to extend from the present back to about 30–50 Ma ago. During this period extensive rifting occurred in several areas of Venus and volcanism has continued at a reduced level relative to the earlier parts of the Guineverian period. Our observations favor a model in which the observed part of the geologic history of Venus (which is the last 20% or less) started with catastrophic tectonic deformation and volcanic resurfacing, followed by the period of declining surface activity of endogenic origin which lasts until now. The lack of surface units representing the first 80–90% of the history of Venus is remarkable from the standpoint of its present low level of activity. This contrast, together with the emerging geologic history of the last 10% of the lifetime of Venus, suggest that catastrophic and/or episodic global processes may have characterized Venus in its earlier history. This factor may also provide an insight into earliest Earth history.

Triassic–Jurassic megasequence stratigraphy in the Northern North Sea: rift to post-rift evolution

Analysis of any sedimentary basin infill should involve an initial separation of the syn-rift and post-rift segments of the succession, particularly where a main aim is to improve prediction of lithology in space and time. The drainage and sediment dispersal patterns and, therefore, the resulting lithology distribution, is usually much more complex in the syn-rift than in the post-rift interval. A varied analytical approach to sequence stratigraphy may therefore be required, especially to honour the local tectonic effects in rifted terranes. Although tectonics are likely to make a fundamental contribution to stratigraphic sequentiality in both settings, the intensity, frequency and area-specific nature of faulting, subsidence and uplift during crustal extension should weigh heavily in the analysis of syn-rift successions. The syn-rift early Triassic basins of the Northern North Sea are compared with the same basins’ mid-Triassic to late-Jurassic post-rift evolution. Further analysis of the post-rift interval shows that the basin had by no means a quiet or uniform development in this phase, but saw the periodic out-building of some 9 major clastic wedges or megasequences, from the Norwegian and Scottish hinterlands. Megasequence boundaries have been chosen to coincide with peak transgression (marine basin) or peak retrogradation (non-marine basin), such that the megasequences have a regressive-to-transgressive structure. This is justified on the grounds of objectivity and simplicity, and the wish to identify individual megasequences with the descriptive and easily understood concept of a clastic wedge. Megasequences have thicknesses between 100 and 1200 m, are time-stratigraphic units of 6–18 Ma duration and should not be confused with higher-order, transgressive-to-regressive sequences. The lower boundary of any post-rift megasequence is a time-line, and will not follow, except locally, a lithostratigraphic horizon. Nevertheless, the lower boundaries can be related loosely to established lithostratigraphic units in the Horda Platform and East Shetland Basin as follows: Scythian (lower Teist Formation) Late Carnian (within middle Lunde Formation) Late Rhaetian (uppermost Lunde Formation) Mid-Sinemurian (within Amundsen Formation) Pliensbachian (within Burton Formation) Early Toarcian (base Drake Formation) Latest Aalenian (lowermost Rannoch Formation) Early Bathonian (lowermost Heather Formation) Late Callovian (middle Heather Formation) The occurrence and broadly similar geometry of the megasequences in both continental Triassic and marine Jurassic successions strongly suggests that post-rift sequentiality was not merely caused by eustatic sea-level changes, but probably included a major contribution from subsidence-rate variation (increased subsidence rates inducing a new megasequence boundary). The main value of mapping the geometry and distribution of the megasequences lies in achieving a better prediction of lithology distribution in the main reservoir fairways, and in creating a basis or framework for analysis of higher-order sequences.

Genetic Sequence Relationships of Winnipegosis Platform Carbonates, Southern Elk Point Basin, North Dakota: ABSTRACT

Examination of cores and well log data from the Winnipegosis Formation (Givetian) within a study area of approximately 11,500 mi/sup 2/ (30,000 km/sup 2/) in northern North Dakota allows recognition of seven time-stratigraphic progradational units within the Winnipegosis Formation. Together with the underlying Ashern Formation, these units are arranged in landward-stepping, vertical stacking, and seaward-stepping geometric patterns, which reflect changes in relative sea level. Abrupt juxtaposition of shallow over deeper water lithologies, evidence for subaerial exposure, and onlap geometries further suggest that these progradational units form two larger, Vail-type sequences separated by regionally persistent unconformities or their correlative conformities. Sea level rise during the early Eifelian caused southeastward onlap of the Ashern Formation onto Middle Silurian carbonates of the Interlake Formation. Maximum flooding, expressed by deepest marine facies and a hardground surface, suggests the existence of a condensed section at the top of the Ashern Formation. This was developed during the maximum rate of sea level rise. A decrease in the rate of sea level rise resulted in aggradation of lower Winnipegosis units on a gently dipping ramp. These are represented by nodular and burrowed open marine limestones with scattered stromatoporoid patch reefs and grainstone shoals. During the subsequent seamore » level fall, represented by Temple units, a shelf margin with pronounced depositional topography and adjacent starved basin were developed. Temple strata include coral-brachiopod-stromatoporoid reefs and productive fore-reef talus deposits along the shelf margin rim.« less

The chemistry of lake sediments in time and space

Five short cores (1.0–1.5 m) representing different depositional zones of an isolated bay of Lake Minnetonka, Minnesota (USA), were independently dated by 210Pb and pollen analysis and were analyzed stratigraphically for elemental chemistry (following sediment fractionation) and sedimentary pigments (including myxoxanthophyll and oscillaxanthin). Because of good dating control, short-interval timestratigraphic units could be traced across the basin, and lake-wide accumulation rates and concentrations could be estimated. The accumulation and concentration of each component at a core-site relative to basinwide averages provide new indices, ‘relative accumulation’ and ‘relative concentration’, that are used to evaluate three major processes controlling sediment deposition: (1) the basin-wide flux, (2) the physical redistribution of bulk sediment, and (3) the selective bio-geochemical transport and fossilization of individual sedimentary components within the lake.

Mesozoic basin evolution in Southern England

Basin evolution is illustrated as a series of structural contour maps, isopachyte maps and cross-sections based upon data from more than 80 borcholes. The maps illustrate the basinal geometry for individual rock units and thus complement existing accounts that are largely based on time-stratigraphic units.

Punctuated Aggradational Cycles: A General Hypothesis of Episodic Stratigraphic Accumulation

According to the hypothesis of punctuated aggradational cycles presented here, the stratigraphic record consists of small-scale (1-5 m thick) shallowing-upward cycles (PACs) separated by surfaces marked by abrupt change to deeper facies. This motif, pervasive in time and environment, is produced by relatively long periods (tens of thousands of years) of base-level stability punctuated by geologically instantaneous relative base-level rises of at least basin-wide extent. As basin-wide lithologic time-stratigraphic units, PACs are fundamental to all aspects of stratigraphic analysis including correlation, paleoenvironmental interpretation, and paleogeographic reconstruction. Of several possible allogenic mechanisms for a pervasively cyclic stratigraphic record, glacial eustasy driven by orbital perturbations is preferred. The PAC hypothesis is presented as a comprehensive model to be tested and modified by examination of the stratigraphic record from an episodic perspective.

Depositional Patterns in Point Lookout Sandstone, Northwest San Juan Basin, New Mexico: ABSTRACT

The Point Lookout Formation, which is well exposed along the northwest margin of the San Juan basin in northwestern New Mexico, includes nearshore sediments deposited during a regression of the Late Cretaceous epicontinental sea in earliest Montanan time. The unit is composed of sandstone and siltstone with sand percentage increasing up-section. Principal outcrop lithofacies include a lower interbedded, highly bioturbated, very fine sandstone and shale that represents the transition from innershelf to shoreface environments. The middle part of the unit gradationally overlies the lowermost lithofacies and represents a complex depositional history in the nearshore zone. These progradational units are thickly bedded and coarsen upward. This simple sequence is interrupted by he occurrence of hummocky stratified storm deposits and by broad surfaces of nondepositional scouring in the lower shoreface. A medium-grained upward-fining sandstone lithofacies that caps the entire formation has an erosional base and large-scale lateral accretionary bedsets. Measured sections from the outcrop of the Point Lookout closely correspond with electric-log patterns from subsurface data east of the outcrop belt. The mapped distribution of SP-pattern facies (representing sandstone textural characteristics) depicts the primary depositional elements of the progradations. Correlation of genetically related sand packages permits the evaluation of changing sedimentation patterns through time. Seven regressive events (time-stratigraphic units) are recognized based on subsurface identification of the transgressive boundaries that rise stratigraphically away from the basin and obliquely traverse lithofacies boundaries. Each unit is composed of three depositional phases (progradation, transgression, and aggradation) that occur in regular succession. Discrete distributary and interdistributary areas were maintained in the initial depositional phases throughout the history of the Point Lookout. In the broad areas between depositional axes the shoreline prograded by the seaward accretion of beach ridges until sediment sources became insufficient to maintain the shoreface advance. Transgressive reworking of the seaward part of the unit followed and dominated the arrangement of net-sandstone thicks by redistributing the sands into a strike-alignment. Each time-stratigraphic sedimentary unit is therefore the product of a progradational-transgressive depositional couplet. Whereas periodic transgressions were mainly erosive, they did cause the formation of coalesced shallow-shelf bars analogous to estuarine-shoal retreat massifs found on the modern continental margin of the Mid-Atlantic Bight. During periods of shoreline stability following transgression a End_Page 449------------------------------ channeled estuarine system developed landward of the retrograded shoreline trend. During this tide-dominated aggradational phase, channels migrated over the back-barrier area and produced the relatively coarser sand facies capping the Point Lookout in the study area. After the estuarine system was infilled, coastal plain facies were established in the former back-barrier zone and progradation was renewed. With the repetition of this depositional pattern through time, the coastal plain advanced in a step-wise fashion. As a consequence of the progradational-transgressive cyclicity, a significant degree of stratigraphic rise was attained during the Point Lookout regression. Each time-stratigraphic coastal sand body acts as a discrete reservoir that interfingers landward with impermeable sediments of the coastal plain facies. Given the necessary present-day structural configuration, major stratigraphic rises corresponding to aggradational phases can act as updip migration boundaries to gas derived from the center of a basin. End_of_Article - Last_Page 450------------

Glacial stagnation-zone retreat in New England: Bedrock control

Glaciofluvial and glaciolacustrine sediments in New England have been grouped and mapped in time-stratigraphic units called out wash sequences (also morphologic sequences and morphosequences). This sequence concept is, in part, a response model complementary to a process model known as stagnation-zone retreat, both originally proposed in 1941. It is suggested that the operating mechanism for this apparently discontinuous type of marginal retreat, whereby the active continental glacier periodically sheds a fringe of stagnant ice, is by downwasting to a critical thickness below which valley ice tongues can no longer support ice movement. At this point the marginal shear zone shifts up glacier, and a new outwash gradient is developed, creating a new outwash sequence. This process is dependent on suitable bedrock topography, which accounts for its prevalence in New England. This mechanism also helps to account for the general absence of ice-contact stratified drift on higher topography and for the observation that outwash sequences are generally not clearly correctable between valleys. For this paper, morphosequences are termed “suites,” so that the term “sequence” can be reserved for a series of suites along a drainage pathway. Parallel suites occupy adjacent valleys and may be time-equivalent. Recognition of serial suites (sequences) and parallel suites will encourage the study of depositional systems over broader regions. A system of tributary sequences occupying a single drainage basin is a sequence net. Because of the tributary arrangement of sequences, the suites within them may be either simple or bifurcating. There are two mechanisms for developing serial suites. Stagnation-zone retreat alone generates a shingled sequence, wherein suites share a common base level but originate from relocated heads of outwash. An outlet sequence, on the other hand, is generated primarily by sequential relocations of base-level thresholds. Outlet sequences also commonly have displaced heads of outwash.

Middle East: Stratigraphic Evolution and Oil Habitat: DISCUSSION

The paper, Middle East: Stratigraphic Evolution and Oil Habitat, by R.J. Murris (AAPG Bull. v. 64, p. 597-618) is discussed. Problems with the time-stratigraphic units used in the article are pointed out, along with the source rocks of the petroleum deposits, the depositional cyclicity, subsidence and sea level fluctuation, and the Middle East geosyncline. (JMT)

Foraminiferal Paleontology and Paleobathymetric Interpretations of Lower Claibornian Rocks of Inner Coastal Plain of North Carolina: ABSTRACT

Outcrop samples of the Castle Hayne Limestone and the Castle Hayne and New Bern Formations and equivalent subsurface rocks yielded 188 species of Foraminifera. Planktonic Foraminifera indicate that these carbonate units are of middle Eocene age and were deposited during the time represented by the upper Globigerinatheka subconglobata (P11) and Morozovella lehneri (P12) Zones (44 to 46.5 m.y.B.P.). This section is bounded above and below by depositional hiatuses; it is overlain by Oligocene and younger rocks and underlain by Paleocene and older rocks. Previous studies have included some of these bounding units as part of the Castle Hayne Limestone. The section studied: (1) correlates with the lower part of the Claiborne Group of the Gulf region; (2) is defined as an inform l time-stratigraphic unit; (3) is herein designated the lower Claibornian unit to distinguish it from Castle Hayne s. l.; (4) does not contain internal hiatuses of significant duration. Cluster analysis of samples of the lower Claibornian unit indicate benthic foraminiferal biofacies representing inner, middle, and outer shelf depositional environments. Paleobathymetric reconstructions utilizing these interpretations indicate that this unit was deposited on an actively block-faulted sea floor. End_of_Article - Last_Page 1664------------

Simulation Model for Petroleum Exploration and Its Applications: ABSTRACT

A computerized simulation model was constructed to synthesize the processes of petroleum generation, migration, and accumulation under relatively simple conditions. The model successfully simulated petroleum accumulation in an existing anticlinal gas field, and was used to estimate the possibility of fault and stratigraphic entrapment nearby. A geologic cross section of the area is divided into a series of vertical columns that are sectioned into rectangular cells representing successive intervals of time and corresponding strata. Four geologic processes are sequentially performed on each cell or pair of adjacent cells. First, sediment is deposited in a cell with its original thickness restored by removing the effects of compaction (deposition). Then, for each time-stratigraphic unit, the compaction caused by increasing time and depth of burial is calculated; the system also estimates the amount of petroleum generated, assuming it to be a function of temperature (compaction and petroleum generation). Primary migration is assumed to occur when petroleum saturation of source beds exceeds the residual amount normally present n thermally mature shale. Secondary migration is assumed to result from buoyancy alone; any petroleum which exceeds the hydrostatic trapping capacity of the shale seal is either allowed to migrate into a cell located along some upward path or escape to the surface (petroleum migration). The model was applied to the anticlinal East Niigata field, Japan, using carefully selected input parameters. Results made it possible to estimate migration paths and timing of entrapment in each producing zone. The model may also be applied to exploration problems. For example, it was used to estimate the possibility of petroleum entrapment in strata near the East Niigata field. Results of this experiment show that the simulation method is potentially useful for estimating the possibility and places of entrapment, especially for stratigraphic traps. End_of_Article - Last_Page 964------------

Mineralogy and Pore Fluid Geochemistry of Great Salt Lake, Utah: ABSTRACT

Piston cores from the South Arm of Great Salt Lake, Utah, have been analyzed chemically and mineralogically. The major detrital minerals are quartz and clay minerals (illite, smectite, mixed layers, and minor kaolinite) with lesser amounts of feldspars and volcanic ash (both felsic and basaltic). Carbonate minerals include aragonite, calcite (0 to 15% MgCO3), high-Mg calcite (50% MgCO3), and dolomite. The carbonate minerals reflect detrital input as well as authigenic growth in the lake. Saline minerals are halite, mirabilite, and gypsum. Pore fluids were extracted from 7 piston cores and subjected to wet chemical analyses. The major ion profiles are similar and cut time-stratigraphic units of the cores indicating little influence from primary formation waters. Instead, compositions are controlled primarily by fluctuations in lake water compositions and mineral reactions. Major ion concentrations are determined by mass transport from the lake waters and from the most soluble minerals present in the sediment. Presence or absence of halite influences Cl- profiles, while SO42- is largely controlled by mirabilite dissolution. Because mirabilite and halite solubilities are nearly equal in Great Salt Lake brines, Na+ is affected by both phases. Gypsum is present; however, because o the much higher solubility of mirabilite, the effect of gypsum on SO42- is obscured, while Ca2+ responds to gypsum. The effects of carbonate mineral reactions are not apparent in the Ca2+ profiles. Because of the relatively small carbonate alkalinity with respect to other weak bases, reactions involving carbonate minerals are difficult to identify. End_of_Article - Last_Page 996------------

探鉱のためのシミュレーションモデル 原理および東新潟地域への応用

A computerized simulation model was constructed to synthesize the processes of petroleum generation, migration and accumulation under relatively simple conditions. The model was successfully simulated accumulation in an existing anticlinal gas field, and was used to estimate the possibility of fault and stratigraphic entrapment nearby.The geological cross section of the area is divided into a series of vertical columns, which are sectioned into rectangular cells representing successive intervals of time, and the strata deposited therein. Four geological processes are sequentially performed on each cell or on each pair of adjacent cells. First, sediment is deposited in the cell, with its original thickness restored by removing the effects of Compaction (Deposition). Then for each time-stratigraphic unit, the system calculates the amount of compaction caused by increasing time and depth of burial; also the amount of petroleum generated, which is assumed to be a function of temperature (Compaction and Petroleum Generation). Primary migration is assumed to take place when the petroleum saturation of the shale source beds exceeds the residual amount normally present in thermally mature shale. Secondary migration is assumed to result from buoyancy alone; any petroleum which exceeds the hydrostatic trapping capacity of the shale seal migrates into a cell located along some upward path or escapes to the surface (Petroleum Migration).The model was applied to the anticlinal East Niigata Field, Japan, using carefully selected input parameters. Results from the model made it possible to estimate the migration paths and the timing of entrapment in each producing zone under the assumed conditions. The model may also be applied to exploration problems. It was used to estimate the possibility of petroleum entrapment in homoclinal strata near the East Niigata Field under several assumed geological situations. Results of this experiment show that the simulation method is potentially very useful for estimating the possibility and places of entrapment, especially for stratigraphic traps.