Detailed electric log correlation, supplemented by cores, in the Eocene Wilcox Group and the Oligocene Frio Formation of the Texas Gulf basin contradicts a commonly held notion that changes in log character across growth faults exclusively reflect changes in environment. An invariable consequence of growth faulting is thickening of a depositional unit in the downthrown block, reflecting a greater subsidence rate. The growth ratio (downthrown or upthrown) varies from just over 1:1 to as much as 10:1.

Analysis and mapping of log character indicate that the basic unit of both deposition and physical correlation is regressive coarsening-upward sequences. In Wilcox deltas, prodelta shales pass up into delta-front sandstones while in Frio barrier-bar or standplains, shelf and lower-shoreface deposits pass up into upper-shoreface sandstones. Regressive packages grade downdip in environment from delta plain and bay or lagoon to offshore marine. Growth faults had no significant surface expression and did not separate contrasting environments.

A marked change in log character (e.g., from smooth to serrated) across a growth fault in a regressive shoreface sequence appears to indicate that the subsidence rate of the downthrown block exceeded a threshold value, enabling preservation of low-energy muddy layers and possibly episodic waning-flow storm deposits that were largely destroyed by fair-weather wave reworking on the upthrown block.

This concept has implications both to regional stratigraphy and reservoir properties. Correlative units can abruptly change log character across growth faults, impeding correlation. Sandstones in the downthrown

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block may contain shale barriers to vertical fluid flow if the threshold subsidence rate was exceeded.

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