Tertiary Reef and Associated Limestone Facies from Louisiana and Guam

Abstract

Similarities in fossil content and lithology of reef and associated limestone facies from southern Louisiana and Guam indicate that biologically similar reefs existed in these areas during middle Tertiary time. The reefs of southern Louisiana are found in the subsurface Heterostegina zone of late Oligocene or early Miocene age; the outcropping Guam reefs are considered to be of early Miocene age. The geologic settings of the reefs differ greatly. The Louisiana reefs are found localized on piercement salt-dome structures and as an extensive reef complex developed on a shallow-water shelf area in southeastern Louisiana. On Guam the flanks of an intermittently active volcano and a ridge of folded volcanics were the sites of reef growth. Six limestone facies are common to these two reef complexes: (1) reef-wall, (2) reef-breccia, (3) off-reef, (4) fore-reef de-trital, (5) fore-reef transitional, and (6) basinal. Material from Middle East reefs of Eocene and Oligocene age is presented for comparison. Reconstructions of depositional environments of the southern Louisiana and Guam reefs are based on stratigraphic, structural, and facies analyses. Present-day counterparts of the Louisiana fossil reefs are submarine topographic prominences on the Continental Shelf off Louisiana and Texas; the Guam fossil reefs are considered analogous to the reef around Borabora in the Society Islands. The authors conclude that Tertiary reefs contain diagnostic limestone facies in predictable spatial relationships and that the nature of the substratum and the local geologic settings do not appear to be controlling factors, provided that ecological conditions are favorable.