Geology of the strata exposed on the Florida Escarpment

Abstract

Three sampling and observational transects were made during five dives in the research submersible Alvin on the steep lower flanks of the Florida Escarpment from < 2500 m to its base (> 3280 m). Twelve other dives were conducted at the base of the escarpment which provided additional rock samples and observations. The face of the escarpment consists of steep to vertical limestone cliffs, with intervening, gently sloping, sediment-covered terraces. Flat-lying beds ranging up to several meters in thickness are truncated at the face along joint planes. Sixty rock samples were collected on submersible transects up the face of the escarpment together with fifteen additional samples from near the base of the escarpment. The sampled strata, which contain sparse early Aptian and late Cenomanian nannofossil floras, are dominated by limestone facies which indicate lagoonal or low-energy intertidal depositional environments. These facies are indicative of deposition at some distance from the original platform edge, suggesting that the original shallow-water Cretaceous margin has been lost to erosion. Although erosion is suggested by the exposure of truncated, nearly horizontally bedded, interior-platform limestone facies at the escarpment face, the data do not indicate when or how much erosion has occurred. However, the exposed surfaces of most rocks are not heavily corroded or bored. Many of the rock samples have smooth surfaces and angular edges. Rocks are covered with a veneer of iron and manganese oxides which are typically < 0.1 mm thick on the lower steep face of the escarpment. Paleogene and most Miocene nannofossils are missing in the pore fillings of the sampled Mesozoic strata. Although 55% of the samples contained biologically penetrated surfaces, the frequency of rock surfaces that are biologically penetrated decreases with depth. The truncated outcrops display fresh surface characteristics which suggest that episodic physical collapse of the lower escarpment face progressively exposes fresh rock surfaces. Strata with weathered surfaces and more than 0.1 mm of FeMn coatings are more common at shallower depths. Thus, the face of the present escarpment is not a preserved Mesozoic platform flank, but a morphological feature which is undergoing episodic active erosion.