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Stratigraphic Architecture of an Incised Valley Fill Documented From Subsurface Data Acquired for Groundwater Remediation, Mobile, Alabama, USA

Shultz, Michael R.*1; Cramer, Rick 1
(1) Environment, AECOM, Oakland, CA.

Gamma-ray logs and lithologic data acquired from wells installed as part of groundwater remediation activities at a former chemical plant near Mobile, AL were used to develop a sequence stratigraphic framework for improved hydrostratigraphic site characterization. The site is located within the greater Mobile Bay Incised Valley System on a terrace located at approximately 10 meters above current mean sea level. The stratigraphic section consists of a basal, sand -rich section approximately 10 meters thick overlying a silty clay section with erosional unconformity. This section contains gravel at its base and is interpreted as a fluvial deposit overlying a sequence boundary. This unit is overlain by a thin, distinctive clay observed in both gamma ray and boring logs. This clay is locally eroded and represents an initial flooding surface developed as relative sea level rose and the bay line migrated landward of the site. Overlying this thin clay is a heterogeneous estuarine unit consisting of clay, silty clay, peat, and laterally discontinuous channel-fill sand bodies, culminating with a high-gamma clay representing a marine flooding surface. This surface is in turn overlain by a coarsening-upward sequence of mud, silt, and fine-grained sand deposited as a bay-head delta prograded seaward. This entire stratigraphic sequence is approximately 25 meters thick and is interpreted as the fill sequence of an incised valley eroded during a relative sea-level lowstand. A second incised valley eroded into the incised valley fill.

Relatively close well spacing at this site (1's to 100's of meters) provides an opportunity to document lateral relations, facies, and dimensions of estuarine channel and bayhead delta deposits at a scale seldom possible with conventional oilfield subsurface datasets. This work highlights the need for stratigraphic interpretation to improve groundwater remediation site characterization and monitoring, and identify potential contaminant transport pathways to ecological receptors. This work also demonstrates the value of groundwater remediation data to augment subsurface data obtained from hydrocarbon exploration and production wells to characterize sedimentary deposits.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California