Palaeoecology of corals and rudists in mixed volcaniclastic–carbonate small-scale rhythms (Upper Cretaceous, Jamaica)

Abstract

The Guinea Corn Formation (central Jamaica, West Indies) is represented by alternating platform carbonates and volcaniclastic sandstones and siltstones and represents the most landward shift of facies in an Upper Campanian? to Maastrichtian transgressive–regressive cycle. The succession consists of 2–30-m-thick rhythms comprising a lower volcaniclastic siltstone/sandstone division; a middle volcaniclastic siltstone/limestone with abundant corals or the rudist Antillocaprina stellata; and an upper division of rudist rudstones and floatstones. The lower division consists of either bioturbated sandy siltstones with lignite and abundant gastropods and infaunal bivalves (lower rhythms in the Guinea Corn Formation), or interbedded volcaniclastic siltstones and normally graded sandstones. The rudist limestones are parallel-bedded and consist of high-abundance mono- to paucispecific assemblages. The boundary between the middle and upper parts of rhythms contains a concentration of oncoids with coral nuclei and a mantle of microbial laminates, red algae and serpulids. Four coral assemblages are recognised based on diversity and coral morphology. The Paracycloseris-? Dasmosmilia assemblage occurs in the lower division of the rhythms and is interpreted as a soft-substrate assemblage that was adapted to elevated nutrient levels. The Ovalastrea-Actinacis, Actinacis-Multicolumnastrea and Actinacis-Calamophyllia-Gyrodendron assemblages occur in the middle parts of the rhythms and consist of bedded rudstones, rudstone mounds and rarer platestones, pillarstones and mixstones. Polyparia arrangements, colony morphology and low diversity suggest the Ovalastrea-Actinacis and Actinacis-Multicolumnastrea assemblages were adapted to high sedimentation rates. The more diverse Actinacis-Calamophyllia-Gyrodendron assemblage is interpreted to have grown under more optimum conditions (low sedimentation rates and high light intensities). Rudists are classified on their growth orientation (elevator, clinger, recumbent) and on their communal relationships (isolated or clustered). The cluster elevator Biradiolites mooretownensis occurs in the lower division of rhythms and the recumbent A. stellata occurs in the middle part of rhythms. The upper part of rhythms contains abundant rudists including isolated elevators ( Antillocaprina occidentalis), cluster elevators ( Bournonia spp., Biradiolites jamaicensis, Thyrastylon spp. and Chiapasella radiolitiformis) and clingers ( Plagioptychus spp.), with the topmost part contains the large recumbent Titanosarcolites.