Modern Sedimentation and Sediment Distribution in the Gulf of Mexico

Abstract

TGS recently completed high-resolution multibeam sonar mapping of the entire Gulf of Mexico, deeper than 750m water depth (total area surveyed = >750,000 km2). Bathymetric data are binned at 15m, and backscatter (the amplitude of the returned sonar signal) maps are mosaicked at 5m, independent of water depth. These data reveal a previously unknown drainage patterns with complex flow patterns and histories, and strikingly different basin floor fan geometries. Sedimentation in the deep GoM is dominated by 3 sources: 1) Mississippi River. The Mississippi River is the 5th largest drainage basin, by area (~3 million km2), in the world and extends about 3700 km from its headwaters to the its mouth. The submarine extension of the Mississippi drainage basin into the Gulf of Mexico extends an additional 1100 km in a sprawling and ill-mannered distribution of channels and fans. In the central US GoM, the Mississippi delta forms a southerly radiating distributary channel network. Farther east, the Mississippi-derived fans turn eastward and impinge on the Florida Escarpment. Farther south, the Mississippi drainage spills southward into the deep basin in Cuban waters and turns westward into the Mexican deep GoM basin. Here, the terminus of the Mississippi drainage in is well imaged with multibeam backscatter in a series of submarine fans with distinctive dendritic patterns. Sub-Bottom Profile (SBP) data indicate that the dendritic backscatter pattern may be related to the base-of-fan channeling ~14m below the seafloor. 2) Rio Grande. The Rio Grande drains a much smaller area (~470,000 km2) and has a length of about 3000 km. The submarine extension of the Rio Grande extends an additional 550 km into the GoM but is much more well behaved than the Mississippi fan system. Until recently, the Rio Grande ponded sediments on a submarine salt terrace just below the shelf edge. Recently, probably in the past few million years, the Rio Grande fan filled its terrace setting and began spilling into the deep GoM basin. East of the Mexican Perdido fold belt we identify at least 3 generations of the Rio Grande fan. The fan emanates from a canyon ~50km SE of the previously identified fan termination. The fan then extends an additional ~230 km SSE across the lower slope basin floor to the seaward-most lobe identified on backscatter. The total distance from the mouth of the present-day Rio Grande River to the most distal mapped lobe is 480km. Two younger progressively backstepping fans are visible in the backscatter and SBP data. On the uppermost fan, the largest axial channel is up to a kilometer wide and 20 to 40m deep. Sands from Jumbo Piston Cores are moderately well sorted, fine grained, quartzose with potassium feldspar, plagioclase, and granitic rock fragments. Paleo drainage reconstruction on the Rio Grande Hinterland point to inclusion of granitic terrains with average drainage distance to the coast of about 1,600km since the Pliocene. Significant reorganization of drainage divides resulted in enlargement of the Rio Grande catchment on the one hand and unroofing exposed the granitic rocks to erosion hence sediment transport on the other hand. There was no major lateral shift of the Rio Grande catchment apex other than the longitudinal due to seaward migration of the coastline from Pliocene to Quaternary. Calcium, Feldspar, clay minerals, accessory minerals and quartz dominate the mineral composition of the Quaternary Rio Grande catchment sediments. 3) Campeche Escarpment. The Campeche Escarpment is a steep slope between 1000 m and 3500 m water depth. The escarpment forms steep (5⁄ to 85⁄) cliffs exposing early Tertiary and Mesozoic strata, including the KT boundary. During low-stands of sea level, short river systems dominate sedimentation in the deep GoM, but during high stands, slumping along the escarpment provides sediments throughout the deep basin. From paleo drainage reconstruction on the Campeche hinterland it is deducted that calcium and dolomite rich sediments dominate the Quaternary Campeche Escarpment whereas calcium, Feldspar and clay minerals dominate (minor fractions of accessory minerals and quartz) the mineral composition of the Quaternary Sur-Este-Campeche catchment sediments on the southside. Longer river systems prevailed on the south side without any major bedrock change from Pliocene to Quaternary. About 1500 sediment cores were collected with 3” diameter 6 m core barrels, and 135 Jumbo Piston Cores (JPC) were collected with 4” diameter 20 m core barrels. The JPC cores were analyzed logged, split, photographed, and sampled for biostratigraphy and C14 dating. We were expecting the very deep-water GoM to be above the Carbonate Compensation Depth (CCD) and dominated by hemipelagic rain accumulating at slow sedimentation rates. This was not the case. We found the deep-water GoM to be a much more dynamic and complex world. 1. Wholescale zones of CaCO3 dissolution with no foraminifera present for radiocarbon dating. 2. Transported extrabasinal turbidites from the Rio Grande and Mississippi Rivers draining the North American Craton extended far into the GoM basin yielding higher than expected low-stand sedimentation rates. 3. Transported intrabasinal slump generated deposits originating from slope failure of the Campeche Escarpment north of the Yucatan Peninsula extended most of the way across the deep water GoM basin. 4. Substantial sea bottom surface erosion was discovered in slope settings and other areas with ages as old as 35,000 years at the sea bottom. Hydrological and hydro-statistical analyses on the digital submarine drainage networks reveals relatively unstable morphological conditions of many submarine catchments throughout the GOM. Morpho-tectonic structures mapped on the same multibeam data also point to very recent deformation of the subsurface. Further structural information on the subsurface obtained magnetic and gravity lineaments explain in a more regional sense the structural setting of seabed structures and drainage networks and drainage characteristics.

AAPG Datapages/Search and Discovery Article #90330©2018 AAPG Hedberg: Geology of Middle America – the Gulf of Mexico, Yucatan, Caribbean, Grenada and Tobago Basins and Their Margins “Integration of the geology of the area from the southern onshore of North America to the northern onshore of South America”, Siguenza, Guadalajara, Spain, July 2-5, 2018