GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 52-4
Presentation Time: 2:15 PM


BROCK, Jonathan E., Biology, Geology and Environmental Science, University of Tennessee at Chattanooga, 615 McCallie Avenue, MC 2653, Chattanooga, TN 37403 and HOLMES, Ann E., Biology, Geology and Environmental Science, University of Tennessee, Chattanooga, 615 McCallie Avenue, MC 2653, Chattanooga, TN 37403,

A section was measured along the Lighthouse Cave trail to the parking area on Dixon Hill to determine the vertical extent of a coarse-grained unit identified previously near the Lighthouse cave entrance in a study focused on general cathodoluminescence and petrography of San Salvador stratigraphy completed in 2016. Unlike other strata on San Salvador, this unit is composed primarily of marine red algae fragments and whole foraminifera (0.6–1mm) with few ooids. We find that the unique layer extends up the trail from Lighthouse Cave entrance toward Dixon Hill lighthouse near 18-meter elevation, making the unit at least 15-m thick.

The measured section began near the Lighthouse cave mouth, sampling at 1.5-m intervals along the trail using a 1.5-m Jacob staff and rudimentary sighting device. This unit is present from the cave mouth to approximately the 17.4 vertical meter mark, and is recognizable in both hand samples and thin sections. Within the 14.3 to 17.4-meter interval, the grain sizes of the foraminifera and red algae fragments become slightly smaller (0.2-0.6mm). At 17.4-meter elevation along the section, algae and foraminifera grains are abruptly replaced by well-formed ooid grainstones (0.2-0.4mm), which continue to the top of the trail.

We postulate that this unit formed as sea level fell following the MIS 5e high stand of sea level, which was responsible for the development of the Cockburn Town reef facies. This marine incursion onto San Salvador likely created shallow, normal marine environments, which supplied the abundant biotic clasts present in this coarse-grained facies, and were slightly reworked during sea-level fall. This unit was subsequently draped by younger, carbonate ooid grainstones that are exposed along the road up to the lighthouse and drape the coarser-grained strata. Future research will determine the extent of this layer throughout the Dixon Hill area and the potential that it forms the core of other island ridges. We will also begin foraminifera identification to obtain potential paleoecologic and biostratigraphic information.

This easily identified unit is unlike other samples of the currently accepted formations and their members. Our data support the inclusion of an additional member within the Grotto Beach Formation, and thus providing a more detailed stratigraphy of San Salvador.