Date of Award

1989

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Clyde H. Moore, Jr

Abstract

St. Croix is a sedimentary island at the juncture of two northeastern Caribbean tectonic provinces. For this reason, the sedimentary development of St. Croix is of considerable importance to regional tectonic reconstruction. This dissertation utilizes evidence from outcrops and samples from an extensive drilling program on St. Croix, and addresses three major subjects: (1) the biostratigraphy and correlation of the Tertiary section, (2) the sedimentological and tectonic development of the Tertiary basin, and (3) the diagenesis and dolomitization of the St. Croix carbonates. Previous models of the late Tertiary development of St. Croix presume a static, isolated land mass, with a self-contained sediment source. These models also suggest that parts of the lower Neogene carbonate section were deposited in shallow water. However, subsurface evidence requires significant modification of previous models of basin evolution on St. Croix. Benthic-foraminiferal faunas collected during drilling suggest that the lower Neogene section of St. Croix was deposited at depths of 600 to 800 m; there is no indication that any part of the present basin was deposited in shallow water. Furthermore, the subsurface evidence indicates that portions of the Neogene section were deposited prior to graben formation, and that faulting began no earlier than the middle Miocene. Therefore, a sediment source external to the present structural basin is required to deposit the pre-graben strata. It appears likely that St. Croix has migrated and was uplifted in the late Neogene, with motion dominated by oblique left-lateral slip accompanied by block faulting. The localized distribution of St. Croix dolomite suggests that dolomitization was restricted to a relict Tertiary lagoon. The isotopic composition of the dolomite is enriched in $\sp{18}$O, and depleted in $\sp{87}$Sr relative to unaltered Pliocene or younger seawater. In addition, the strontium isotopic composition is incompatible with the Pliocene age of the dolomitic host rock. In order to account for the geochemistry and geologic distribution of the dolomites I suggest that dolomitization took place from fluids that were produced from a mixture of evaporated seawater and groundwater similar to that on St. Croix today.

Pages

312

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