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Abstract: Precambrian Tidal Facies Sequence: Roraima Group, Venezuela

Santosh K. Ghosh

The Proterozoic Roraima Group displays sequences, structures and association of tidal deposits on an extensive marine shelf. They may serve as useful analogs for exploration in tide-dominated facies. These deposits occur in isolated table mountains extending over 200 km in eastern Venezuela.

The strata reveal intertidal fining-upward, and more dominant subtidal channel and bar deposits. The stacked intertidal sequences consist of, from base upward: Lithofacies (1)--alternations of thin horizontally laminated (1A) and larger cross-bedded units (1 B);

Lithofacies (2)--alternating thinly cross-bedded and horizontally laminated units; and

Lithofacies (3)--rippled and flat laminated silty fine sandstone with flat-topped and double-crested ripples. The subtidal assemblage consists of lithofacies 4 and 5.

Lithofacies (4a)--fine-grained sandstone with cosets of trough cross-strata. Unidirectional westerly directed currents formed the sinuous crested dunes with foreset wavelengths up to 50 m. Hummocky cross-strata, convolute lamination and overturned foresets are common.

Lithofacies (4b)--medium grained lens-shaped tabular avalanche foresets with diagnostic lateral sequence, intraset disconformities and back-flow ripples along high-angled foresets. Parallel and ripple-laminated units occur above and below individual avalanche foresets.

Lithofacies (5)--medium grained tabular cross-stratified sandstone consisting of single or cosets of large tabular (planar, tangential or sygmoidal) cross-strata. It shows 2-m-thick foresets in channel-form units associated with lithofacies 4. Bipolar herringbone cross-strata (ENE-WSW), lateral sequences, reactivation surfaces and soft sediment structures are characteristic. The texturally mature Roraima sediments contain little shaly intercalations and clay drapes.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela