SEDIMENTATION ASSOCIATED WITH OPENING OF THE ATLANTIC OCEAN: DETRITAL ZIRCON DATA FROM THE SUBSURFACE LUCULA FORMATION, OFFSHORE WEST AFRICA

The early syn-rift Jurassic Lucula Formation of offshore Cabinda, West Africa, includes lacustrine fan-delta and lacustrine shoreface facies, which were likely sourced by regional river systems draining axially into a graben associated with initial opening of the south Atlantic ocean basin. Using the Sensitive High Resolution Ion Microprobe-Reverse Geometry (SHRIMP-RG) at the Stanford-U.S.G.S. Microanalysis Center, we analyzed 30 detrital zircon grains for each of two Lucula Formation sandstone samples taken from separate borehole cores spaced 25 km apart, in an attempt to constrain sediment dispersal patterns and unroofing history of the early south Atlantic rift system. Isotopic analysis of both samples produced primarily concordant ages with distributions dominated by “Pan-African” ages, here defined loosely to include ages ranging from 600-450 Ma. One sample is 67% composed of Pan-African grains and 23% composed of Neoproterozoic-latest Mesoproterozoic grains (897-1050 Ma), with the remaining 10% of the grains Neoarchean in age (2650-2748 Ma). The age distribution of the other sample is 80% Pan-African grains, with the remaining 20% composed of one grain of each of the following age: Silurian (365 ±2 Ma), Devonian (422 ±3 Ma), Neoproterozoic (960 ±8 Ma), Mesoproterozoic (1517 ±15 Ma), Paleoproterozoic (1922 ±15 Ma) and Neoarchean (2377 ±8 Ma).

Lucula strata overlie what is believed to be mainly Paleoproterozoic with lesser Archean West African crystalline basement; however, both Lucula samples lack abundant Paleoproterozoic and Archean grains. In addition, the absence of Late Paleozoic zircon grains indicates that igneous rocks in the Upper Carboniferous-Jurassic Karoo Supergroup, which crop out in the Cassanje Graben of central Angola, along with any syn-rift volcanic rocks, were not a major contributor of detritus. Instead, these data indicate initial syn-rift detritus originated from Pan-African rocks, potentially due to reactivation of Pan-African mobile belts as zones of rifting to form either conduits for sediment transport and/or localized sediment sources, or due to a prevalence of Pan-African zircon grains in Paleozoic West African cover strata that has since been eroded away.