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Late Pleistocene Rio Grande and Bryant Fans: Two Unique Deep-Sea Fan Types in the Northern Gulf of Mexico and Their Implications for Petroleum Systems


Detailed studies of the late Pleistocene Rio Grande and Bryant fans in the northern Gulf of Mexico reveal two unique types of deep-sea fans with morphologies and architectures different from typical modern fans such as Mississippi Fan. Rio Grande Fan developed a unique distributary channel system on a wide terrace on the middle continental slope. Multiple slope canyons fed numerous upper-fan channels which appear to be a braided or anastomosing planform. These channels coalesce into three large channels that extend down the middle-to-lower fan. Eventually they merge into one channel, at the distal end of the fan, that flows down through the Perdido Canyon. The channels display low sinuosity in general, are largely erosional in nature, and do not have aggradational channel-levee systems. Sandy piston cores, prolonged 3.5 kHz echo character, and a braided channel system all suggest the Rio Grande Fan is a sand-rich, deep-sea fan. In contrast to Rio Grande Fan, Bryant Fan is a moderate-sized, mud-rich fan. The modern channel at the surface is a single, meandering, aggradational, channel-levee system that extends the length of the fan and ends in a single lobe. The fan consists of two primary growth units that functioned similarly to the modern channel on the fan surface. Each unit has a series of older, buried, single or individual channel-levee systems that appear to traverse the entire fan to reach individual depositional lobes on the distal outer fan. Piston cores show muddy sediments with numerous silt/sand beds typical of levee deposits. Although there is moderate meandering of the modern channel, there does not appear to be a significant amount of channel migration and bifurcation. Moreover, there is no evidence that channel avulsion similar to Mississippi Fan and other typical deep-sea fans (e.g., Amazon, Indus fans) has occurred on Bryant Fan. Similarly, the buried channel systems in the growth units appear to be similar to the modern channel: each channel appears to extend the length of the fan without channel switching through avulsion. The Rio Grande and Bryant fans provide modern analogues for older, productive fans in the northern GOM. For example, the Rio Grande Fan may be a modern analogue for the Oligocene fans of the Frio Formation, and Bryant Fan is an excellent modern analogue for the Miocene MCAVLU Fan.