GRAIN-SIZE AND PETROGRAPHIC ANALYSIS OF PALEOGENE AQUIA FORMATION SEDIMENTS AND INDURATED BEDS IN THE NORTHERN SALISBURY EMBAYMENT: TINKERS CREEK, MARYLAND – MID-ATLANTIC COASTAL PLAIN

As part of the Atlantic Subsurface Stratigraphic Initiative, Paleogene outcrop and core material were examined to determine regional lithologic and stratigraphic boundaries in the Salisbury Embayment. To help achieve this goal, grain size and petrographic analysis of Paleogene sediments and indurated beds from a continuous section in Maryland were analyzed to provide regional sedimentological context. The Tinkers Creek locality is in Prince George’s County and consists of approximately 24 ft of an apparently continuous section of the Aquia Formation along the creek at Valley View Community Park. The lowest part of the Aquia Formation at Tinkers Creek (Sample TC1-1) is located beneath an indurated ledge that forms a small waterfall/chute in the creek and is possibly Selandian in age, based in part on the frequent occurrence of calcareous nannofossil Heliotrochus kleinpellii and on its stratigraphic position in the section. The first occurrence of Heliolithus riedelii is only 0.5 ft above the basal sample and marks the transition from Zone NP6 to Zone NP8. It is unclear whether the absence of Zone NP7 is due to a hiatus or because the paleoenvironment on the shelf was not conducive to the propagation of discoasters (indicators for Zone NP 7). There are two laterally continuous indurated beds that occur at the Tinkers Creek location. Observational petrography of thin sections from these indurated beds indicates that the dominant framework grains are subangular to subrounded, monocrystalline quartz, rounded to globular glauconite, bivalves, benthic and planktic foraminifera, and minor crinoids. The matrix consists of <5% micrite. Cement is non-ferroan, fine-grained to sucrosic, primary and secondary calcite. Porosity is nonexistent. Grain size analysis of the Aquia Formation sediments indicates an overall coarse lithology with average percentages of 5.0 for clay, 36.5 for silt, and 58.0 for the sand-size fraction. The Aquia at Tinkers Creek consists of very fine to fine, subrounded to subangular quartz grains with glauconite at 10-20% and mica at 1-2%. The lithostratigraphic and petrographic features identified in this investigation will be examined for potential use in correlation across the Salisbury Embayment.