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Early Diagenetic Nodules, Compaction, and Secondary Lamination in Early Jurassic Lacustrine Black Shale, Culpeper Basin, Virginia

Pamela J. W. Gore

Ostracodal limestone nodules in black lacustrine shale of the Midland fish bed, Culpeper basin, contain relict precompactional textures. Laminae in the shale fan break apart, and grade laterally into and deflect around the nodules. The nodules are elongate, and range from 5 to 16 cm wide and 4 to 9.5 cm thick. Petrographically, the nodules range from massive or vaguely layered, pelleted, ostracodal grainstone, to structure grumeleuse, to mud containing diffuse laminae of crushed and oriented ostracode valves, recording successive stages of compaction. The nodules grade laterally and vertically into black, laminated, fossiliferous shale. The gradational relationship with shale and the range of textures indicate that nodule growth began before appreciable compaction occurre , and continued during early stages of compaction. The nodules are associated with elongate fragments of coalified wood near the base of the fish bed, which are interpreted as fallen branches and trees. Nodules probably formed as a result of early diagenetic cementation in an alkaline microenvironment adjacent to decaying wood. The change in character of the sediment with compaction, as preserved in the nodules, indicates that laminations in the Midland fish bed are diagenetic in origin. It also indicates that black shale can originate from the compaction of relatively massive, bioturbated, pelleted, ostracodal grainstone.

The amount of compaction can be determined by comparing the thickness of nodules to the thickness of laterally equivalent black shale. Compaction ratios range from 3:1 to 5:1, but because compaction accompanied nodule growth, these ratios are probably underestimated.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.