Predicting and characterizing the subsurface remains a key endeavor for applied geoscientists. Biostratigraphy provides critical support through its ability to support correlation within the subsurface and to determine depositional environments of sedimentary rock units. A combination of time-based correlation and paleoenvironmental interpretation under the umbrella of sequence stratigraphy helps organize stratigraphic information and allows for predictions to be made as to the sedimentary successions and their character that can be expected between data-points and in data-poor regions. Issues that can be typically resolved by biostratigraphy occur at a wide variety of scales and – in decreasing order of scale – include generation of play concepts and global- and basin-scale play screening through support of correlation, Earth systems science context (e.g. paleoclimate, eustasy), gross depositional environment mapping, and sequence stratigraphy; age frameworks for basin modelling; appraising assets; and the effective, efficient and safe exploitation of assets through operational biostratigraphy including casing/coring picks and biosteering. Applied biostratigraphy has supported the ongoing efforts of the oil and gas industry for over one hundred years and the lessons from these endeavors can be applied to alternative subsurface use cases, including storage, defining optimal locations for low-carbon energy sources such as geothermal, and for engineering projects associated with, for example, wind-farm location. Importantly, biostratigraphic data is suitable for assisted interpretation arising from the digital revolution, meaning that data gathering and interpretation are becoming faster and better integrated.
The upper Claiborne, Jackson, and Vicksburg groups record approximately 13 Ma of deposition within the upper Paleogene section of Mississippi (middle Eocene through lower Oligocene). These strata are characterized by repeated suites of siliciclastic and carbonate lithologies, including sandy, glauconite-bearing shell beds and marls exposed updip (in outcrop), and thick limestones that developed across a downdip paleo-high. These carbonate strata are well known from subsurface mapping efforts, but their significance remains largely unexplored in outcrop-based stratigraphic and paleontological studies. Here, we review and integrate subsurface data from well log and core studies with field-based observations to propose a new, mixed carbonate-siliciclastic depositional model for the upper part of the Paleogene section of Mississippi. Deltaic strata overlie widespread basin-margin unconformities but thin distally to condensed marine shales across the Wiggins Arch. Accordingly, these beds reflect lowstands in sea level and were deposited through the regressive outbuilding of the shoreline into the Mississippi Interior Salt Basin. Glauconitic shell beds occur in association with coastal barrier systems, where they reflect the transgressive reworking of previously deposited sand and bioclastic material. In contrast, carbonate sediments are interpreted to have formed in response to the onshore trapping of siliciclastics, coupled with enhanced carbonate production during relative highstands in sea level. These are characterized by successive foraminiferal-algal grain banks in the subsurface, which thin and grade landward to mollusk-rich shelf mudstone-wackestones (marl and limestone units) in outcrop. Notably, both carbonates and glauconitic shell beds appear to broadly record marine transgression but are expressed differently based on relative shoreline position within major base-level cycles. This model differs from the predominantly siliciclastic style of deposition that characterizes the present-day northern Gulf margin and appears to lack a strong modern analog. It also provides a basis for refined stratigraphic interpretation, with important caveats to sequence stratigraphic models commonly applied in this section, and establishes a framework in which to analyze late Paleogene faunas in the context of evolving, process-related depositional environments.
Dinoflagellate cyst palynology and lithostratigraphy from three continuously cored boreholes (Blackbird, Smyrna, and Woodland Beach), supplemented by calcareous nannofossil biostratigraphic data, were used to refine the age and correlation of three Paleocene-Eocene formations in northern Delaware: the Vincentown, Manasquan, and Shark River. Lithologic logs reveal significant facies changes in the upper Paleocene Vincentown Formation over a short distance (*9 km) southeastward, transitioning from very permeable glauconite-quartz-shell-rich sands that form the regionally important Rancocas/Aquia aquifer to much less permeable muddy sands and sandy muds. These lithologic changes represent a shift from nearshore, clear, warm-water, carbonate-rich sand shoals to mud-dominated deeper-water, offshore shelfal environments. Dinocyst assemblages including Areoligera gippingensis, Cribroperidinium giuseppei, and Stenodinium meckelfeldensis confirm the Paleogene ages and track these paleoceanographic changes from shallow coastal settings updip to neritic or outer marine settings downdip. The Vincentown Formation is unconformably overlain by the lower Eocene Manasquan Formation, which consists predominantly of silt and clay with relatively consistent lithology across the study area, representing general neritic or open marine conditions. Lithology, dinoflagellate cyst assemblages, and calcareous nannofossil data indicate that the Marlboro Clay and its record of the onset of the Paleocene-Eocene Thermal Maximum (PETM) global warming event is absent from the Paleocene-Eocene transition at these sites, with an unconformity marking the boundary between the Vincentown and Manasquan formations. The youngest unit examined, the Eocene Shark River Formation, unconformably overlies the Manasquan Formation and consists of interbedded clayey silts and sands, with dinoflagellate assemblages suggesting a return to coastal or inner neritic settings. Calcareous nannofossil zones recognized from the Woodland Beach core corroborate the depositional ages determined using dinoflagellates throughout the sequence, confirming the chronostratigraphic framework for these important coastal plain units.
This study revises the lithostratigraphic framework of the Upper Devonian interval traditionally assigned to the Greenland Gap Group across the central Appalachian Valley and Ridge Province. The work aims to modernize and standardize lithostratigraphic nomenclature, establish a new reference section and demonstrate how the revised stratigraphy improves edge-matching of 1:24,000 scale geologic maps and supports compilation mapping at scales of 1:100,000 and larger. The revision eliminates the names Greenland Gap Group, Scherr Formation and the Minnehaha Springs Member of the Scherr Formation; reassigns all strata previously designated as Scherr Formation by Dennison (1970) to the upper Brallier Formation; and abandons the basal Mallow Member of the Foreknobs Formation, placing its strata within the upper Brallier as originally defined by Butts (1918). The contact between the Brallier and Foreknobs formations is placed at the base of the first mappable, ridge-forming package of fine-to coarse-grained, cross-bedded, sandstone beds, often containing rounded quartz pebbles with minor interbeds of shale and siltstone. This contact may be gradational in places but, even in absence of good exposure, can usually be distinguished topographically in recently produced lidar-derived imagery as having elevated relief due to the presence of more resistant, compositionally mature coarse-grained sandstone-rich strata. Applying this criterion for mapping the contact between the Brallier and Foreknobs formations has resulted in reconciliation of mismatches of geologic contacts along several 7.5-minute quadrangle boundaries in the states of Virginia, West Virginia, Maryland and Pennsylvania. A new reference section at Baker, West Virginia showcases the contacts between the Harrell Shale, Brallier Formation, Foreknobs Formation and Hampshire Formation. A digital outcrop model of the reference section is provided for future preservation.
This study presents the first detailed biostratigraphic analysis of calcareous nannofossils and Alveolina assemblages from the Mojen section in the Alborz Zone, North Iran. By integrating Alveolina distribution with nannofossil biozones, the research shows that the NP10 (CNE3) to NP12 (CNE4) zones correspond to the Shallow Benthic Zones SBZ8-SBZ10, representing an early Ypresian age (upper middle Ilerdian to early Cuisian). Furthermore, Alveolina citrea (Drobne) has a longer biostratigraphic range in Turkey and Iran, extending into younger strata within the SBZ7-SBZ9 interval and aligning with the NP zones, highlighting its widespread occurrence across the western-central Neo-Tethys.
