Transgressive Lag of Flat Rip-up Clasts — Substratum for Initial Growth of Upper Cambrian Large Microbial Bioherms

Abstract

The discovery of large oil and gas fields in microbial carbonates has motivated studies of key outcrops, as potential reservoir analogs, to understand better the initial establishment, overall evolution, and demise of the buildups. In the past three years, a Rice/Trinity Industry Microbial Research Consortium has analyzed in great detail world class Upper Cambrian microbial reefal outcrops (Point Peak Member of the Wilberns Formation) in Central Texas. A unique carbonate unit, covering as much as 25 km², beautifully crops out in cliffs along the Llano River. This unit consists of multiple 8-15 m-thick and 10-40 m-wide microbial bioherms, linked laterally by a series of inter-reef grainstone beds alternating with thinner silty mostly siliciclastic levels. This unit sharply contrasts with the underlying unit made of alternating mixed carbonate-siliciclastic silty and sandy thin mostly recessive beds, interpreted to record very shallow subtidal and even intertidal marine environments during a general sea level regression. The sharp transition between both units stands out on the cliffs as the most developed overhangs and corresponds to the flat bases of the microbial bioherms and the contemporaneous initial accumulation of a 1-2 m thick inter reef resistant grainstone bed. This bed usually dips towards each bioherm on both sides, thins, and in many instances disappears towards their center. Along the James River, cores were drilled through a very small “embryonic” bioherm and the upper part of the underlying bed. Analyses of the cores clearly show that the microbial growth was initiated on a low relief small pile of flat rip-up clasts of similar lithologies as the bed itself, covering an erosional unconformity. Along Mill Creek, a fully developed 8 m-thick bioherm, rotated 90 degrees and lying on its flank, offers its flat base for observation and sampling. The bioherm detached itself from the nearby cliff where a small depression in the first inter reef bed, often with flat rip-up clasts at its very base, still matches the shape of the rotated bioherm edge. The flat center of the bioherm base consists of a10 cm-thick sheet of flat rip-up clasts, interpreted as a transgressive lag, on top of which the microbial growth was initiated. A core drilled through the very bottom of the bioherm confirms this interpretation; at its very bottom a few small rip-up clasts are mixed with stromatolitic columns.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016