ICHNOFACIES ANALYSIS OF A MIOCENE CARBONATE DISTALLY-STEEPENED OUTER RAMP, SOUTHEASTERN FLORIDA PLATFORM

Seismic-reflection data and data from wells penetrating the southeastern Florida Platform indicate that the early-to-middle Miocene upper part of the Arcadia Formation consists of three prograding, distally-steepened, carbonate ramps dominated by a heterozoan-particle association. The oldest ramp is overstepped by the overlying ramp, whereas the youngest ramp shows backstepping followed by the progradation towards the underlying ramp margin. Each ramp is an individual depositional sequence that consists of high-frequency cycles (HFCs). To understand the succession of depositional environments composing the individual HFCs it is important to identify the vertical ichnofacies succession of each. Identification of the ichnofacies is challenging because many of the trace fossils in slabbed core samples are poorly visible. Digital-image treatment of core jpeg images from a single HFC was used to improve trace-fossil visualization and ichnofacies characterization. The treatment was conducted using image-editing computer software and a recent method successfully applied to support the ichnofacies characterization of modern deep-sea cores. Particular modifications of several image adjustments, including brightness and vibrance levels, were incorporated into this first application on carbonate rock cores, resulting in an improved trace-fossil visualization. A fining-upward HFC, bounded by marine erosional surfaces, and underlain by a Glossifungites Ichnofacies was delineated, spanning the upper part of a transgressive systems tract of the youngest ramp. Within the HFC there is a shift upward from a distal expression of the Cruziana Ichnofacies to an archetypal CruzianaIchnofacies. This indicates a shift upward from relatively deep to shallower outer-ramp environments approximating the maximum flooding surface of the depositional sequence that is equivalent to the youngest ramp. These data inform the regional stratigraphic conceptualization of the intermediate confining unit in southeastern Florida.