Talk Title: Unstable Deltas: Why, What Happens, and Exploration Significance
Speaker: Larry Meckel
Date: May 20, 2005
Publication: The Outcrop, May 2005, p. 5
Deltas that prograde into deep water produce distinctive depocenters which are important exploration targets at the shelf edge. The deltas become unstable as they prograde into deep water depositing units that are geologically unique and very different from their counterparts that prograde across a stable shelf or shallow seaway.
Karl Terzaghi, a soil mechanics engineer who studied modern deltas, provided a reasonable explanation for why these deltas are unstable. He documented theoretically and confirmed by direct observation of deltas that the stability of a delta is related to the length of the foreset, thus the depth of water it progrades into. Two well studied modern deltas have reached deep water at the present day shelf margin: The Balize (Birdfoot) Mississippi Delta and the Fraser River Delta. They are both very unstable and are characterized by a variety of contemporaneous (early) structures including growth faults, numerous other associated faults and/or diapirs. They are also actively supplying sediment basinward through gully systems to deeper water along the margin of the delta.
Long continuous cores (300 feet) and high resolution seismic in the Late Pleistocene Lagniappe delta offshore Alabama add more insight to the story and provide unexpected positive surprises for exploration.
Virtually all the largest clastic plays (in terms of reserves found) in the onshore and shelf parts of the Gulf of Mexico in the last 30 years have been in these unstable shelf margin delta systems and their linked deep water facies. Significant new discoveries in these deltas continue today, the result of new concepts applied to a very mature basin.
So, where would one go in the Cretaceous seaway to apply these concepts?