Abstract

Stacked and laterally offset clinoform packages associated with lowstand deltas comprise the northeastern Gulf of Mexico shelf edge, from the modern Mississippi River delta to DeSoto Canyon. Within the late Pleistocene Lagniappe delta complex (offshore Mississippi-Alabama), thick sandy prograding clinoform bed sets display strong impedance contrasts on high-resolution seismic, suggesting significant free-gas content. Carbon-14 dating places the Lagniappe delta at the shelf-slope transition about 1 ka before the latest Pleistocene glacial maximum. Thin heterolithic clinoform toes extend down slope to a channel-levee system that feeds bypass sediment to a deep slope fan. In sedimentary units that contain both fluids and gases, laminated sand, silt, and clay units create effective capillary seals that inhibit vertical hydrocarbon migration while focusing lateral and up-dip transport. Significantly, Lagniappe delta clinoform toes extend down slope into the gas hydrate stability zone, which may act as a regulator of up-dip hydrocarbon migration.

In response to the ca. 100 ka glacioeustatic cycles of the Pleistocene epoch, reduction in hydrostatic pressure and warmer water periodically impact the top of the gas hydrate stability zone, triggering hydrate instability. Rapid decomposition of large gas hydrate reservoirs can dramatically reduce sediment strength and thereby lead to slope failures and canyon/gully formation. Reentrants in the continental margin formed by this process are exploited by falling-to-lowstand deltas as catchments for clinoform toes and by-pass routes for sediment transport to deep-water settings. At more modest rates of gas hydrate decomposition, gas is forced into surrounding sediments where it migrates up-dip along capillary seal limited porous and permeable pathways provided by the heterolithic clinoform toes. It appears that this process provides an early gas charge to the shelf-edge deltas.

Under rising-to-high sea-level conditions and rebuilding of the gas hydrate reservoir, Loop Current intrusions can raise bottom water temperatures on the upper slope (< 1000 m) by 3-10 degrees C, causing near surface gas hydrates to dissociate and, making gas available for up-dip migration. Loop Current impacts are also important for periods of lowered sea level. Presently, gas seeping from truncated clinoforms on the shelf, and ¹³C-depleted authigenic carbonates in Lagniappe delta cores suggest that hydrocarbon migration is an on-going process at the shelf margin.