AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Forced Regression Versus Drift Sedimentation – Clinoform Stacking Patterns in Carbonate Platforms (Maldives, Indian Ocean)
(1) Dept. of Geosciences, Hamburg University, Hamburg, Germany.
(2) Dept. of Earth Science MS-126, Rice University, Houston, TX.
(3) Dept. of Sedimentology and Marine Geology, VU University Amsterdam, Amsterdam, Netherlands.
New multichannel high-resolution seismics acquired in 2007 (Cruise M74/4 of the RV Meteor) from the Maldives carbonate platform (Indian Ocean) show that a prograding complex of clinoforms, characterized by offlapping geometries, and previously interpreted as deposits formed during a forced regression, is a body of drift sediments.
The Maldives archipelago is a 3 km thick isolated carbonate platform consisting of 2 rows of atolls enclosing the basin of the Inner Sea. The platform margins facing the open Indian Ocean exclusively aggraded, whereas the margins facing the Inner Sea alternately aggraded and prograded. Imaged bank growth during the Early to late Middle Miocene can be subdivided into 3 seismic units, based upon the stacking pattern of 11 individual seismic sequences. An initial aggrading and prograding phase is followed by a dominantly aggrading growth pattern, showing a successive steepening from a ramp-like configuration to a rimmed platform. The third unit consists of dominantly prograding sediment packages, showing an alternation of oblique-tangential and sigmoid prograding geometries. Seismic units 2 and 3 are interpreted to reflect a complete lower-frequency, higher-order sea-level cycle, in which the general switch in platform mode, attributed to a maximum flooding, coincides with the transition from the Lower to Middle Miocene. During the late Middle Miocene, the Inner Sea bank margins followed distinct growth patterns. Synchroneity of partial bank drowning and localized deposition of sediment-drift bodies on the one hand, as well as continuous bank growth on the other hand, is interpreted to show a turnover from sea-level governed platform-margin development to a mixed sea-level- and current-driven stratal packaging.
Drift deposits are characterized by gently, basinwards dipping large-scale clinoforms, which internally show low-angle truncations, offlapping geometries and wavy seismic reflections indicative of submarine dunes. The turnover in bank-margin development is attributed to the onset or intensification of the monsoon and related currents during the Neogene. This case study implies that a downward shift of clinoform growth and offlapping geometries in carbonate platforms are not necessarily indicative for a sea-level driven forced regression. Findings are expected to be applicable to other examples of Tertiary platforms in the Indo-Pacific region.