Abstract Since the publication of Shaw’s Time in Stratigraphy in 1964, paleontologists of Amoco Corporation have concentrated on developing a Phanerozoic biostratigraphic database for use in graphic correlation applications and composite standard development. This developmental phase brought the company to a position where it can routinely utilize its composite database to solve problems of correlation related to hydrocarbon exploration and production. This experience shows that two kinds of composite standards maximize data value. These are (1) Worldwide and (2) local composite standards. A Worldwide composite standard develops a comprehensive fossil occurrence database that establishes an accurate understanding of the global ranges of fossil species. Fossil datums are placed into a chrono - sequence scaled to a framework of Worldwide reference sections. With database evolution through graphic correlations, traditional zonal resolution is usually exceeded due lo the factoring of all fossil data, not just zonal markers and familiar accessory forms. Paleontologists in industry commonly must use multisource, multivintage data of varying qualities within the same project. Systematic comparisons of such information to a Worldwide standard are particularly beneficial in overcoming the incongruities of multiple zonal schemes and varied taxonomies that make up such datasets and which tend to confound traditional biostratigraphic analysis. They also are helpful in identifying and removing spurious data. A worldwide composite standard may distort the value of local species ranges linked to regional paleoenvironmental and geographic restrictions. These foreshortened ranges may otherwise be very valuable in establishing intrabasinal correlations. For these reasons, the concept of a “local” composite standard developed within Amoco to maximize basin evaluations and regional stratigraphic applications. A local composite standard captures the regional biostratigraphic signincance of all fossil occurrences, including those localities calibrated initially to the Worldwidestandard. These calibrated localities have lines of correlations that place a local fossil sequence within a worldwide context. The resulting database provides optimum refinement for regional correlations, while resolving important basinwide time-stratigraphic breaks undetected by purely local zonations.

Abstract Stratigraphic analysis of sedimentary basins is critical for correlation in a basin, for reconstructing the geohistory of a basin, and for developing a successful petroleum exploration strategy for a basin. In studying only the shelfal areas of basins that are characterized by carbonate or mixed carbonate and siliciclastic deposition and in which stratal patterns are driven by low-frequency, tectonic-eustatic events, a stratigraphic analysis based on the cyclicity recorded in the strata (transgressive-regressive cycles) has utility for correlation, for geohistory interpretation, and in formulating petroleum exploration strategies. This is the case for the Cretaceous section in basins of the northeastern Gulf of Mexico. In utilizing the concept of transgressive-regressive (T-R) cycles, eight T-R cycles are recognized in Cretaceous (upper Valanginian to lower upper Maastrichtian) strata of the northern Gulf of Mexico. The cycles consist of a transgressive (aggrading and backstepping) phase and a regressive (infilling) phase. These T-R cycles are useful for intrabasin correlation of Cretaceous strata in the Mississippi interior salt basin and for interbasin correlation of Cretaceous strata in the Mississippi interior salt basin and the East Texas salt basin. Six regional Cretaceous unconformities and associated hiatuses (late Valanginian, middle Cenomanian, late Turonian to middle Coniacian, middle Campanian, late Campanian to early Maastrichtian, and late Maastrichtian) and nine regional transgressive events (early Aptian, early Albian, middle Albian, early Cenomanian, late Cenomanian to early Turonian, late Santonian, early middle Campanian, early late Campanian, and early Maastrichtian) have been identified as major events in the geohistory of these basins. Hydrocarbon production from Cretaceous reservoirs in the northeastern Gulf of Mexico can be categorized as to the phase of T-R cycles. Sandstone reservoirs associated with the early transgressive aggrading phase have accounted for 42% of the 7.437 TCF of natural gas produced from Cretaceous reservoirs in this region. Sandstone reservoirs associated with the late transgressive backstepping and regressive infilling phases have accounted for 63% of the 2 BBO produced from Cretaceous reservoirs in this region. These findings indicate that the primary natural gas exploration target in the northeastern Gulf of Mexico should be Cretaceous sandstone reservoirs of the transgressive aggrading phase of T-R cycles and that the principal oil exploration target in this region should be Cretaceous sandstone reservoirs of the transgressive backstepping and regressive infilling phases of T-R cycles. These exploration targets are associated with structural traps related to salt movement.

Abstract Refined intrabasinal correlation of medial-Silurian strata has led to recognition of discrete eustatic and tectonic controls on the stratigraphic architecture of the Appalachian foreland basin. Major unconformities and disconformities are used to define third- and fourth-order sea-level cycles (depositional sequences and sub-sequences). Although they are asymmetric, most unconformities are present along both basin margins, and their timing corresponds with sea-level lowstands in other Silurian basins. The correlation with apparent, global, sea-level lowstands suggests a eustatic component to Silurian Appalachian basin unconformities. Silurian sequences are divisible into systems tracts that are correlatable across the basin. Transgressive systems tracts are laterally correlative, retrogradational, carbonate and sandstone successions that onlap unconformities on northwestern and southeastern margins of the basin, respectively. Highstand systems tracts are thickest in the basin center and thin laterally toward each margin. They are divisible into early highstand phases, typified by aggradational, fine-grained siliciclastic and argillaceous carbonate successions, and late highstand (or regressive) phases, that characteristically exhibit a general upward-coarsening (progradational) pattern. These regressive deposits are typically divisible into two or more sub-sequences whereas the transgressive and early highstand systems tracts comprise a single subsequence. Smaller-scale, discontinuity-bound, stratal packages that are interpreted to record fifth- and sixth-order sea-level changes, are analogous to parasequences sets and parasequences, respectively. Many of the small-scale cycles can be mapped basin-wide or unti 1 they are truncated under marginal unconformities. High-frequency, eustatic, sea-level changes or climatic oscillations are plausible mechanisms to explain the pervasiveness of these cycles. Documented tectonic controls on stratigraphic architecture include lateral migration of the foreland-basin axis and uplifts along both the cratonic arch and the tectonically active eastern basin margin. These tectonic signatures are imprinted on marginal unconformities and are also recorded by progressive lateral shift in the locus of thickest accumulated sediment and deepest facies. Tectonically imprinted unconformities are more fully developed on one basin margin and are distinguished from purely eustatically generated unconformities by their asymmetry. Consequently, large-scale sedimentary cycles, that are bounded by unconformities and correlate with sea-level changes in other basins, record an interplay between foreland basin geodynamics and eustatic processes of similar rate. In contrast, smaller-scale, sedimentary allocycles, that have recurrence intervals that outpace rates of foreland-basin flexure, more clearly record high-frequency, low-amplitude, eustatic sea-level changes or climatic oscillations.

Abstract In the central and western part of India, the Neoproterozoic deposits are identified in the Vindhyan and Marwar Basins. The Vindhyan Basin consists of two sub-basins; one in the eastern part and the other in the western part. The basic problem with the Vindhyan Basin is the correlation of the eastern part with the western part, as the two areas show different stratigraphic successions and the outcrops in the eastern part are not traceable in the western part. In this paper, an attempt is made to suggest intrabasinal correlation within the Vindhyan Basin on the basis of stromatolites, carbon isotope data, microbial mats, fossils and lithology. The Marwar Supergroup is developed in the western Rajasthan and unconformably overlies the Malani Igneous Suite, which has previously been dated as 779–681 Ma. On the basis of the available fossil records, the Jodhpur Group has been assigned an Ediacaran age and the Precambrian–Cambrian boundary is suggested within the Bilara Group. As both the Maihar Sandstone of the eastern part of the Vindhyan Basin and the Jodhpur Sandstone of the Marwar Supergroup have been assigned an Ediacaran age, these have been correlated.