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ABSTRACT: Rhodolite and Encrusted-Grain Sedimentation, Thunder and Lightning Knolls, Southwest Caribbean Sea

Mark W. Peebles, Pamela Hallock, A. C. Hine

Thunder and Lightning Knolls are the smallest of seven carbonate banks comprising the western half of the Nicaraguan Rise, a large tectonic-structural feature separating the Cayman Trough from the Colombian basin. The study area is located between the exits of two open seaways that define the eastern and western margins of larger Arawak

Bank. Topographic upwelling and current velocities of up to 100 cm/sec are the consequence of constriction of the Caribbean Current as it flows northward over the Nicaraguan Rise. The knolls have an average depth of approximately 30 m, with margins steeply sloping to a channel depth of 200 m.

Bank-top and margin sediments are coarse, especially on the windward margins. Grains and pebbles larger than 1 mm are encrusted by a variety of organisms, including red algae, foraminifera, serpulid worms, and sponges. Pebbles include rhodolites and aggregate grains bound together by encrusting organisms.

Rhodolites have been suggested as possible paleoecologic indicators of energy conditions, temperature, depth, or calcium carbonate supersaturation. Other workers have recently shown that aspects of nodules commonly used as indicators (e.g. shape, growth form) have little predictable relation to specific environmental conditions in the eastern Caribbean.

Vertical accretion of Thunder and Lightning Knolls is not keeping up with recent rates of sea level rise. Rhodolite formation and heavy encrustation may be products of nutrient levels intermediate between nutrient deficiency that promotes high carbonate production and buildup and higher nutrient supplies that completely suppress carbonate buildup.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990