The Latemàr—A
Matter Of Competence
Egenhoff, Sven O.1, Arndt
Peterhaensel2 (1) Colorado State University, Fort Collins, CO (2)
Universitaet
The bucket morphology characteristic for
many modern tropical carbonate platforms develops, if the tropical seafloor
moves into and remains long enough within a favorable window where rates of
production of bound carbonate and accommodation space are in balance. “Empty
buckets” form as a consequence of stress-induced strongly deviating growth
between frame-built metazoan margin and starved lagoon. Only glacially-induced
high-amplitude sea-level rises create sufficient accommodation space to enable
their formation. Tropical carbonate platforms during interglacial or greenhouse
times, in contrast, are thought to develop filled bucket morphologies with
metazoan reefs confining a shallow lagoon. We postulate that in the absence of
major frame-builders, for example following global faunal crises, a
microbially-induced and cement-reinforced tepee belt assumes the role of energy
barrier on many ancient carbonate platforms of greenhouse times. The
post-extinction Triassic Latemàr platform in the Italian Dolomites is a case in
point. Competence of its marginal part rendered by intermittent tepee formation
provided protection for incompetent shallow lagoonal material and prevented
wave-driven off-platform transport. An intermittently established hydraulic
head between the ocean and the restricted lagoon supplied large quantities of
marine water percolating from the open sea into the sediment/rock body of the
tepee belt. Moving along boundaries of cm- to dm-scale layers, these fluids
produced ample amounts of fortifying early cements. In contrast to the
conventional model, this relief- and margin-maintaining process is active
during phases of falling accommodation. Once established, the tepee belt
becomes an integral part of platform geometry buffering the impact of sea-level
changes and subsidence.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California