SEISMIC GEOMORPHOLOGY AND STRATIGRAPHY OF INCISED VALLEYS CUT BY MEANDERING RIVERS IN THE LATE MIOCENE MISSISSIPPI RIVER DELTA, LA

Incised valleys are significant geomorphic features created in response to base level fall by sinuous channels migrating within a valley. We use a 1400 km² 3D seismic volume located under Breton Sound, LA, integrated with a selection of well logs to characterize the geomorphology and fill of three incised valleys present within the upper 1.5 seconds of the seismic volume. Interaction between growth faults and these incised valleys is also evaluated. Seismic attributes used to image the valleys in planview, as well as provide some lithologic constraint in the absence of well logs, include: a) instantaneous sweetness, an attribute calculated as reflection strength divided by frequency, and b) similarity, an edge detection attribute.

Valley fills are 1 to 5 kilometers wide, 25 to 50 meters thick, and imaged for up to 30 kilometers across the survey. Valleys in planview display a characteristic ‘scooped’ shape, interpreted as the time-transgressive composite erosion associated with laterally migrating channels. One of the valleys appears to have reoccupied the path of an older valley fill. We suggest that subtle valley paleotopography may act as an attractor for future valleys, where background slope is small.

Fourteen wells penetrate incised valleys and show a highly variable fill with thick sand or thick mud, with some fills showing interbedded mud and sand. Well logs in interfluve areas show a dominantly muddy lithology.

The data images approximately 30 growth faults, some of which may have interacted with channels and affected valley morphology. In contrast to the majority of smaller distributary channels found within the survey, incised valleys appear to be steered along or away from growth faults. This observation suggests that faults are able to affect the course of valleys to a greater extent than small channels. We suggest that this is because valleys are long lived features which do not avulse before feeling the effect of shorter time scale faulting events.

This study contributes to our understanding of the geomorphology and dynamics of incised valleys in the subsurface and has important environmental and economic implications. In addition to being a strong indicator of sea level fall, incised valleys can transport large amounts of sand to the shelf edge and can act as hydrocarbon reservoirs when filled.