CORAL–ALGAL AND FORAMINIFERA PHASE SHIFTS IN THE EOCENE FORALGAL REEF AT IODP SITE U1376 (UNIT IIA): ECOLOGICAL AND ENVIRONMENTAL ASPECTS

IODP Site U1376 on the Burton Guyot Seamount in the southern Pacific Ocean encountered a foralgal reef (Unit IIA) between 23.45 and 38.60 mbsf. The 16.3m of this unit consists of a sequence of white crystalline grainstones, wackestones, packstones, and boundstones. The section includes abundant fossils include such as foraminifera, algae, coral, ostracods, mollusks, and echinoderms. Twenty-two thin sections were produced from this interval in order to study the biostratigraphy, paleoecology, and sedimentary response to fluctuating sea-level on an isolated carbonate platform.

Planktonic foraminifera were identified in thin-sections throughout Unit IIA. Ten species were identified in the studied interval. They include Catapsydrax unicavus, Catapsydrax sp., Globorotaloides quadrocameratus, Paragloborotalia griffiniodes, Parasubbbotina eoclava, Parasubbbotina varianta, Globigerina officinalis, Subbbotina eocaena, Turborotalia frontosa and Turborotalia pomeroli. This is a middle Eocene assemblage and may be correlated with the Lutetian biozone E10. Two specimens of macroforaminifera have been identified in the core as Discocyclina marginata and Heterostegina sp. Smaller benthic foraminifera identified include Cibicidoides micrus, Alabamina sp., and Lagena sp. The primary framework builder in this core is the red coralline algae Lithothamnion camarasae occurring in both frondose and crustose forms. A secondary framework builder, the scleractinian Isopora togianensis is also present throughout the section.

Alternations of growth forms of L. camarasae occur as couplets between flooding surfaces with beds containing frondose forms overlain by crustose forms. These couplets are interpreted as parasequences formed during small fluctuations in relative sea level with the frondose forms of L. camarasae growing to fill the available accommodation space as a “catch-up” response to relative sea level rise with the crustose forms representing a “keep-up” phase.

The development of the foralgal reef in IODP site U1376 represents a response to relative sea level change associated with subsiding oceanic crust. Comparison of site U1376 with other know Paleogene foralgal reefs in different geologic settings will provide insight into reef development during periods of global warmth.