A tidal depositional model for the Carolina sand (Pliocene, Talparo Formation) of central Trinidad

Abstract

A BSTRACT. — The Pliocene geological history of Trinidad’s Northern Basin is little known. The need for such knowledge is pressing; some rock units cropping out there yield hydrocarbons offshore in the Gulf of Paria, but the environments in which they were deposited have yet to be deciphered. As a first step towards unravelling this geohistory, this study examines the sedimentology of the Carolina sand, a small and informal member of the Pliocene Talparo Formation. From plan view the lenticular Carolina sand is concluded to comprise a tidal sand ridge that built up almost to sea level. This interpretation is supported by abundant sedimentary structures indicative of tidal influence (herringbone cross-bedding, mud couplets, clay drapes). Drape and ripple orientations show northward transport that is concluded to be indicative of ebb tide currents. For most of the succession, sediments alternate between two lithofacies (tabular cross-bedded sands with clay drapes, and interlaminated sands and silts). Comparison with tidal flats suggests these lithofacies indicate rapid cycling between different water depths. The depth changes are suggested to be related to outflow from the paleo-Orinoco River, which, like the present day Orinoco, is suggested to have raised sea levels around Trinidad during the annual rainy season. The cross-bedded sands were deposited as deeper-water megaripples on the tidal ridge surface during the rainy season, the interlaminated sands and silts under intertidal conditions when sea levels fell during the dry season. The Carolina sand was probably deposited during one of the eustatic sea level highstands hitherto noted farther east, in the offshore Columbus Basin. K EYWORDS. — tidal sand ridge , Orinoco , clay drapes , herringbone cross-bedding , mud couplets