Abstract

There were some significant Valanginian changes in the Gulf of Mexico affecting marine tectonics, terrestrial erosion, and sedimentation on the Florida shelf and in the eastern Gulf of Mexico deepwater areas. New seismic refraction data indicates that termination of Gulf of Mexico sea-floor spreading left its tectonic mark at the beginning of the Valanginian stage (early Cretaceous; Snedden et al., 2013). Subsequent uplift of the Florida Peninsular Arch caused a source terrane to perch above the landscape, readying it for erosion and transport across the shelf and into the deepwater Gulf of Mexico. Coeval supply from an Appalachian source terrane brought additional siliciclastic material into the deepwater Gulf of Mexico off the Florida coast. Our hypothesis is that the Appalachian Mountains fed a westward-oriented sand-rich fan, and, together with the Florida Peninsular Arch, supplied a progradational sandy delta-fed apron that paralleled the present western Florida coastline.

The Gulf of Mexico Basin Depositional Synthesis Project (GBDS) at the University of Texas Institute for Geophysics (UTIG) maintains a database of Gulf of Mexico onshore and offshore wells, well logs, biostratigraphy data, and UTIG and third-party seismic data. Utilizing a 2D seismic dataset over the eastern Gulf of Mexico and the GBDS well database, we interpret a chronostratigraphic surface of Valanginian age across the Florida shelf, north of the Sarasota Arch. We calibrated the well log data with two onshore Florida wells.

A chronostratigraphic interpretation of the top of the Valanginian deposits in the eastern Gulf of Mexico indicates a continuous, sub-horizontal surface across the shelf. The Peninsular Florida depocenter is found to be farther north than previously hypothesized, while still limited by the Sarasota Arch as the southern shelf boundary. Our working hypothesis, based on timing of uplift and subcropping strata, is that the Peninsular Florida source terrane rocks have different textural properties than the Appalachian source terrane rocks. The Peninsular Florida Valanginian rocks, penetrated by the Stanolind-Sun Perpetual Forest #1 well, are fine- to coarse-grained sandstones interbedded with shale. The Appalachian Valanginian rocks, penetrated by the Magnolia State 5B #1 well, are very fine- to coarse-grained micaceous sandstones interbedded with shale (Fig. 1).

The differing rock properties of the Valanginian deposits in the Appalachian and the Peninsular Florida pathways support our hypothesis that two distinct terranes source the progradational sandy delta-fed apron. The micaceous, very fine- to coarse-grained sands of the State 5B #1 well suggest a longer transport distance than the fine- to coarse-grained sands of the Perpetual Forest #1 well. Grain size is an important discriminator and discounts the possibility that two different deltas deposited the material from the same Appalachian source terrane. The dominance of muscovite and biotite in the Appalachian-source rocks is consistent with the metamorphic rocks in the Appalachian Mountains; the Paleozoic basement of northern Florida, on the other hand, is primarily comprised of Paleozoic sandstones, siltstones, and shales, along with either Paleozoic or late Proterozoic volcanic rocks (Heatherington and Mueller 1997).

The exact provenance of the Valanginian deposits would be better constrained by detrital zircon (U–Th)/He and U/Pb double-dating, which is a goal of future research. Previous work by Lisi (2013) suggested that basinward Jurassic deposits of a delta system in the same paleogeographic location as our Valanginian deposits derived from Laurentian and Grenville Appalachian orogenies and a Gondwanan, Pan-African, and Trans-Amazonian/Eburnean tectonic event Appalachian source. Additional detrital zircon work should also be done for the Peninsular Florida source terrane, building upon the U/Pb dating of northern Florida Suwannee Basin wells by Opdyke et al. (1987), Odom and Brown (1976), and Mueller et al. (1994). Valanginian wells, such as Perpetual Forest #1, should yield zircons with U/Pb formation ages of the Paleozoic volcanics and (U–Th)/He exhumation ages of the Valanginian sea-floor spreading termination and Ocala Arch uplift event.

The Travis Peak formation of Texas is time-equivalent to the Valanginian Hosston formation of the eastern Gulf of Mexico. Being an onshore hydrocarbon-rich formation, the Travis Peak is better studied. Ewing (2010) found two discontinuous sand-rich facies in the Travis Peak and suggested a basal lowstand facies and an upper transgressive facies. Our investigation of the Hosston in the eastern Gulf of Mexico identified a continuous, time-stratigraphic surface on the modern shelf rather than a lowstand facies and a main, transgressive facies. The boundary between the Valanginian Hosston and the overlying Sligo is identifiable in 2D seismic data across the shelf and is the basis for our interpretation of only a single systems tract.

Seismic and well log interpretation, with careful attention to biostratigraphic data, and identification of key properties of core, cuttings, and core chips lead us to believe that a Valanginian progradational sandy delta-fed apron is fed by two distinct source terranes. The large sand body could be a hydrocarbon reservoir, forming an attractive trend in this under-explored region of the Gulf of Mexico Basin.