Complex ooids from Triassic lake deposit, Virginia

Abstract

Complex reworked ooids are described from a lacustrine limestone intercalated in the Manassas formation (Late Triassic) near Culpeper, Virginia. These ooids are characterized by several sets of concentric or non-concentric rings with intercalations of an argillaceous calcilutite which also builds their nuclei and the interstitial matrix of the deposit. The nuclei are simple or compound. In the first case, they consist of an irregular pellet probably generated by a process of aggregation of aragonite needles. Compound nuclei display two to sixteen ooids embedded in a variable amount of mud. Both types of reworked ooids result from the numerous repetition of phases of oolitization, deposition in a mud where pellets were formed by aggregation, and reworking. These processes must have taken place in the immediate vicinity of each other. Such conditions are actually very common in shallow water environments, lacustrine, lagoonal, or marine. Complex reworked ooids occur in small number in almost all oolitic limestones but have been frequently overlooked. Their abundance in the Triassic of Virginia is unusual for a carbonate rock. However it is comparable to the situation displayed in many oolitic iron ores where numerous complex ooids indicate reworking phenomena of high complexity.