The Geology of Delaware

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Omar Formation

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Geologic Time Period: 
middle Pleistocene
Coastal Plain - Primarily Surficial Unit

The following description was published in RI76 Stratigraphy, Correlation, and Depositional Environments of the Middle to Late Pleistocene Interglacial Deposits of Southern Delaware, Ramsey, K.W., 2010:

The Omar Formation was originally described (Jordan, 1962) as consisting of interbedded, gray to dark gray, quartz sands and silts with bedding ranging from a few inches to more than 10 feet thick. Thin laminae of clay are found within the fine, well-sorted sands. Silt mixed with sand generally contains some plant matter and where dark in color could be considered organic. Sands contain wood fragments, some of which are lignitic.

On the basis of regional mapping by Ramsey (2010), the description of the Omar Formation is modified from that of Jordan (1962). The Omar Formation consists of quartzose, greenish-gray to light-yellow, homogeneous, fine to very fine sand with scattered medium to coarse laminae commonly overlain by dark-greenish-gray, silty clay to clayey silt with scattered shell beds and bioherms of the oyster Crassostrea. The silty clay is overlain by a light-gray, fine to coarse sand. Coarse sand and gravel interspersed with organic-rich horizons that include stumps and logs of cypress trees is found both at the base of the Omar Formation and at the top of the silty clay.

The Omar Formation ranges from 10 to 80 feet thick. In the western portions of its extent in the vicinity of Cypress Swamp and to the north where it grades into the Lynch Heights Formation, the unit is typically a sheet of moderately well sorted to well sorted, fine to coarse sand.