--> Abstract: Platform Size as a Fundamental Control on Architecture and Sequence Style: Comparison of Triassic Carbonate Platforms of Northern Italy and South China, by D. J. Lehrmann and R. K. Goldhammer; #90937 (1998).

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Abstract: Platform Size as a Fundamental Control on Architecture and Sequence Style: Comparison of Triassic Carbonate Platforms of Northern Italy and South China.

LEHRMANN, D. J., Univ. of Wisconsin-Oshkosh, Geology Dept., Oshkosh, WI, 54901, R. K. GOLDHAMMER, Sonat Oil Co., Houston, TX.

Numerous factors are viewed as basic mechanisms controlling the architecture of shallow-marine carbonate platforms. These include subsidence, sea-level, climate, substrate geometry, slope-angle and organic evolution. Although stark contrasts exist in the architecture of different sized platforms, the size of a platform is rarely discussed as a contributing mechanism.

Middle Triassic isolated platforms from Italy and South China have distinctly different architectures that are best explained to result from differences in scale and the texture of sediment shed from their margins. The platforms in Italy range from a few km to 20 km across. These buildups have a pinnacle-style architecture with grainy interiors and angle-of-repose, avalanche-dominated slopes. The Chinese platform, in contrast, is up to 50 times larger than the Italian platforms. It contains mud-rich lagoon and tidal fiat interior facies and has an architecture with the familiar progression of bank to reef-rimmed to erosional escarpment profiles. The larger size of the platform increased its mud production and protected it from extensive winnowing, which in turn resulted in shedding of muddy sediments that were stable on relatively gentle, basin-margin slopes that progressively steepened and ultimately led to avalanche deposits and a high-relief erosional escarpment. In contrast, the lesser mud production of the Italian platforms forced avalanche and talus deposition to dominate throughout, producing their pinnacle geometries

These differences also controlled sequence stratigraphic styles. In the Chinese platform, the more gradual basinward facies-transitions and gentler slopes allowed sequences to be defined in traditional fashion — using margin geometries. Sequence definition using parasequence stacking patterns, however, was largely unsuccessful, due to random vertical patterns induced by autocyclicity in the interior tidal flats. In contrast, margin geometries were of little use in defining sequences in the Italian pinnacles due to their sharp vertical transitions. The grainy interiors and lack of tidal fiats inhibited autocyclicity, however, allowing a much more orderly record of parasequence stacking patterns and definition of a high-resoulution sequence stratigraphy.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah