ORDOVICIAN SEA STACKS OR SILURIAN CAVES: UNCONFORMITIES AND THE BRASSFIELD LIMESTONE IN INDIANA

The Silurian (Llandovery) Brassfield Limestone at Napoleon, Indiana, exhibits several unique features related to its unconformable relationships with adjacent rock units. The unit is unusually thick (3.5 m) here compared to other localities in the region, and is generally of uniform character, being a yellowish brown, medium bedded, fossiliferous grainstone. The Brassfield is bounded by temporally significant, but relatively planar, unconformities. Marking the Ordovician/Silurian boundary, the base of the unit overlies the highly fossiliferous and argillaceous Whitewater Formation, but throughout most of the quarry the contact is poorly exposed. The Osgood Formation, a well-bedded even textured argillaceous limestone, overlies the Brassfield unconformably.

Extensive quarrying here has revealed rare localized features within the Brassfield that are related to one or the other of these two unconformities. These features range from about a meter in diameter to as much as 3 m high and 7 m wide. Lithologically, these features consist of light grey to reddish grey, dense, medium bedded micritic limestone, which contrasts sharply with the surrounding Brassfield. Commonly, they contain zones of abundant strophomenid brachiopods, many of which occur as imbricated valves standing on edge. The contact between these features and the Brassfield is exceptionally sharp and conspicuous. The base of these features is not exposed, and it is unclear if they rest on basal Brassfield or on the underlying Ordovician. The largest ones are overlain by a half meter or more of Brassfield; small examples seem to be “floating” within the Brassfield. The shape of the larger features is irregular, with a broad base that narrows sharply, more than a meter inward, while the upper beds display either near vertical or overhanging sides.

Originally, these features were thought to represent cave infills related to the Llandovery unconformity. New exposures and additional study, however, suggest that they may represent sea stacks composed of Ordovician rocks and are, therefore, related to the unconformity at the base of the Brassfield. The most important evidence for these micritic features being sea stacks are localized borings filled with possible Brassfield sediments on their surface and the presence of possible Ordovician conodonts.