Biogeographic, stratigraphic, and environmental distribution of Basilosaurus (Mammalia, Cetacea) in North America with a review of the late Eocene shoreline in the southeastern coastal plain

A new specimen of Basilosaurus cetoides was discovered on the banks of the Flint River in Albany, Georgia, USA, in 2010. This fossil, which was the most complete specimen of the species from Georgia to date, consisted of five nearly complete and two partial post-thoracic vertebrae, tentatively identified as S4 through Ca6. During excavation, however, the site was looted and most of the specimen was lost to science. Nonetheless, we use this discovery as an opportunity to update the current state of knowledge on the stratigraphic, biogeographic, and environmental distribution of Basilosaurus in North America, as well as the position of the late Eocene shoreline in the southeastern United States. The results show that Basilosaurus was most abundant across the southeastern coastal plain during the early to middle Priabonian, coincident with the late Eocene maximum marine transgression. The decline in Basilosaurus localities is associated with the retreating shoreline of the terminal Eocene. The majority of Basilosaurus localities fall well south of the position of the late Eocene shoreline hypothesized in this study, suggesting the genus favored middle to outer neritic zones of the epicontinental sea. The comparatively low number of Priabonian specimens in the Atlantic Coastal Plain versus the Gulf Coastal Plain, then, suggests the presence of shallow zones in the Atlantic Coastal Plain that may have limited the distribution of Basilosaurus across the region. The hypothesized shoreline of this study ultimately differs from earlier reconstructions by extending the Mississippi embayment at the Bartonian/Priabonian boundary farther north than previously noted.

Late Cretaceous sediment provenance in the eastern Gulf Coastal Plain (U.S.A.) based on detrital-zircon U-Pb ages and Th/U values

ABSTRACT Detrital-zircon U-Pb geochronology documents a regional- to continental-scale drainage reorganization along the eastern Gulf Coastal Plain (USA) from the Late Cretaceous (Cenomanian) to the Paleocene–Eocene. We present detrital-zircon U-Pb ages and Th/U values from the Maastrichtian Ripley Formation to determine the sedimentary provenance and to provide spatiotemporal resolution of drainage reorganization. The Ripley Formation contains a 12.7% overall average abundance of detrital zircons with low (< 0.1) Th/U values relative to the underlying Cenomanian Tuscaloosa Group (3.6%), the overlying Paleocene–Eocene Wilcox Group (2.8%), an Appalachian foreland composite (2.1%), and the laterally equivalent McNairy Sandstone in the northern Mississippi Embayment (3.8%). Multidimensional scaling of detrital-zircon U-Pb spectra shows that the Ripley Formation is dissimilar from underlying and overlying Gulf Coastal Plain units, the McNairy Sandstone, and an Appalachian foreland composite sample because of differences in proportions of Appalachian (490–270 Ma) and Grenville (1250–900 Ma) zircons. We interpret the southern Appalachian Piedmont province as the principal sediment source region for the Ripley Formation to account for the elevated abundance of grains with low (< 0.1) Th/U values and unique detrital-zircon U-Pb age spectra. Results suggest a regional-scale (105 km2) drainage network, which delivered sediment to the Maastrichtian coast followed by northwestward littoral transport and eventual mixing with Appalachian foreland-derived sediment in the northern Mississippi Embayment. This study further brackets drainage reorganization along the eastern Gulf Coastal Plain and demonstrates how simple chemical–age relationships, such as zircon Th/U values coupled with U-Pb ages, can be used to evaluate sediment provenance.

Detecting lithospheric discontinuities beneath the Mississippi Embayment using S-wave receiver functions

SUMMARY Identifying upper-mantle discontinuities in the Central and Eastern US is crucial for verifying models of lithospheric thinning and a low-velocity anomaly structure beneath the Mississippi Embayment. In this study, S-wave receiver functions (SRFs) were used to detect lithospheric boundaries in the embayment region. The viability of SRFs in detecting seismic boundaries was tested before computing them using the earthquake data. A careful analysis using a stochastic noise and coda model on the synthetics revealed that a negative velocity contrast could be detected with certainty at low to moderate noise levels after stacking. A total of 31 518 SRFs from 688 earthquakes recorded at 174 seismic stations including the Northern Embayment Lithospheric Experiment, EarthScope Transportable Array and other permanent networks were used in this study. Common depth point stacks of the SRFs in 1° × 1° bins indicated a continuous and broad S-to-P converted phase (Sp) arrival corresponding to a negative velocity contrast at depths between 50 and 100 km. The observed negative Sp phase is interpreted as a mid-lithospheric discontinuity (MLD), and several possible origins of the velocity drop corresponding to the MLD are explored. After quantitative analysis, a combination of temperature, water content and melt content variations are attributed to explain the observed MLD in this study. The observations and interpretations in this study support the previous claims of an MLD in the Central and Eastern US and provide a possible mechanism for its origin.

Impact of climate change on groundwater resource in a region with a fast depletion rate: the Mississippi Embayment

Mississippi Embayment (ME) is one of the fastest groundwater depletion regions around the world, while the impacts of climate change on groundwater resources in the region are complex and basically unknown. Using the U.S. Geological Survey's Mississippi Embayment Regional Aquifer Study (MERAS) model, such a challenge was addressed through the base, wet, and dry simulation scenarios. Over the 137-year simulation period from 1870 to 2007, the cumulative aquifer storage depletions were 1.70 × 1011, 1.73 × 1011, and 1.67 × 1011 m3, respectively, for the base, dry, and wet scenarios. As compared with that of the base scenario, the aquifer storage depletions were only 1.76% more for the dry scenario and 1.8% less for the wet scenario. A multiple regression analysis showed that the aquifer storage depletion rate was controlled more by the groundwater pumping and stream leakage rates and less by the groundwater net recharge rate. Groundwater table variation in the forest land was much smaller than in the crop land. Results suggested that groundwater pumping rather than climate change was a key driving force of groundwater depletion in the ME. Our findings provide a useful reference to water resource managers, foresters, and farmers in the ME and around the world when developing their groundwater supply strategies.

Cretaceous cultch: substrate availability for oyster Exogyra in the Maastrichtian of the eastern Mississippi Embayment

The gryphaeid oyster Exogyra Say, 1820, is ubiquitous in Upper Cretaceous sediments in the southeastern United States. Like many oysters (Order Ostreida), Exogyra attached its shell to hard surfaces on the seafloor by means of cementation. Throughout its lifetime, Exogyra may preserve through bioimmuration the characteristics and even skeletal remains of substrate organisms, including mollusk shells, echinoids, and bryozoans. Exogyra costata of all sizes were collected from three different localities within upper Maastrichtian deposits in northeast Mississippi and their bioimmurations analyzed. Substrates were identified and classified to compare the three localities’ substrate taxa in order to probe differences in benthic population structure. The results of this pilot study provide an overview of available surfaces on the Late Cretaceous seafloor on the eastern side of the Mississippi Embayment. The data suggest that taxonomic diversity among utilized substrates may increase from north to south, which corresponds to increasing depth from shallow marine to deeper sediments on the shelf.

Contributions to groundwater from National Forest lands in the Mississippi Embayment: a century-long simulation

Very little effort has been devoted to analyzing the contributions of National Forests to groundwater resources in the US and around the world. In this study, the US Geological Survey's MERAS (Mississippi Embayment Regional Aquifer Study) model was used in the ModelMuse simulating system to estimate more than a century of subsurface hydrologic processes, groundwater budgets, and spatial-temporal groundwater level distributions in three forests in Mississippi, US. The results showed that groundwater recharge and stream leakage are important for groundwater storage in this region. All three forests served as groundwater sinks at times and sources at others, but the volume changes were relatively small. Groundwater levels declined over the simulation period – 1900 to 2014 – beneath all three forests, especially around the DNF (Delta National Forest) where groundwater abstraction is relatively intense. Knowledge gained from long-term hydrologic simulations and water budgets is useful when managing forest land groundwater resources.