zircon geochronology, supplemented by thin section petrography, point counting, and well logs, were used to constrain the provenance of basal Triassic/Jurassic (?) clastic sedimentary rocks in the Eastern Gulf of Mexico (EGOM). Basal clastics were deposited as synrift strata into half grabens present along the EGOM and eastern margin of Laurentia that formed during the initial break up of Pangea. Samples used in this study were collected from the South Florida Basin (SFB) and the South Georgia Rift (SGR). Previous studies of post-rift Jurassic sediment in the Gulf of Mexico has identified a change in source from Laurentian dominated sources in southern Mississippi/Alabama, to Gondwanan dominated sources in the Florida Apalachicola Embayment/offshore. Comparing the provenance of initial syn-rift strata to postrift strata is used to identify initial sediment sources into the EGOM rift basins, and how provenance changed over time. Detrital zircons were collected from five SFB well cutting samples from the Jurassic (?) Wood River Formation and proximal basal sandstones, and six SGR well cutting samples from the Triassic (?) Newark Supergroup, as well as one sample from offshore southern Florida basement. Predominant age populations for the SFB are consistent with a Pan-African origin (525-680 Ma, 1900-2200 Ma). Samples from the SGR are dominated by Laurentian sources (265-330 Ma, 350-420 Ma, 430-490 Ma, 900-1300 Ma), as well as secondary Pan-African sources (525-680 Ma, 1900-2200 Ma). Provenance data is supported by thin section point counting and well logs. Samples in the northern SGR are predominantly derived from Laurentian sources, while the southern samples 3137 and 1854 are a mix of Laurentian and Gondwana signatures. K-S test comparisons of basal sandstones from this study and post-rift Jurassic Norphlet Formation samples from Lovell (2010) and Lisi (2013) show little correlation between samples, implying they do not share similar sources. The exception is the offshore Apalachicola Embayment samples, which indicates that the SGR may have been a potential source for Jurassic sediment in the northern EGOM. The integration of geochronology and petrographic techniques in this study suggests a mix of primarily sedimentary sources from the Suwannee Terranes and metamorphic sources from Laurentia supplied the initial sediment into the EGOM. Results from this study can be used to better constrain tectonic terranes and paleogeography of the early EGOM.

By comparing new detrital zircon provenance analysis of Triassic synrift sediments from the Tallahassee graben (FL), the South Georgia rift basin (GA), and Deep River rift basin (NC) with our previous detrital zircon provenance data for the Jurassic Norphlet Formation erg in the Eastern Gulf of Mexico, we have developed a regional model of Triassic-Jurassic erosion and sediment transport. In the Eastern Gulf of Mexico, detrital zircon ages observed in Triassic synrift clastics from the Tallahassee graben and southern South Georgia rift system contain not only Gondwanan-aged and Grenville-aged zircon grains but also an abundance of Paleozoic detrital zircon grains, reflecting sediment influx from rocks associated with the Paleozoic orogens of eastern Laurentia. Although Paleozoic detrital zircon grains are present in the younger Norphlet deposits, they are less abundant than in Triassic rift sediments. In southwest Alabama, the most abundant detrital zircon age population in the Norphlet Formation is Grenville-aged (950-1,250 Ma). In the Conecuh embayment of southeastern AL and western FL panhandle, Norphlet samples show a marked decrease in Grenville detrital zircon and an increase in 525-680 Ma zircon ages, interpreted to represent influx from rocks associated with the Gondwanan Suwannee terrane. In the Apala-chicola Basin, the proportion of Gondwanan zircon ages increases to nearly 40% of the total population and Grenville-aged grains constitute just ~20% of the population. We suggest that the difference between Triassic and Jurassic detrital zircon signatures in the Eastern Gulf of Mexico reflects significant unroofing of Paleozoic rocks during early Mesozoic rifting of the easternmost Eastern Gulf of Mexico, possibly including rocks equivalent with those exposed in the Talladega slate belt units. Subsequent erosion of rift-flanking highlands to expose older Gondwanan and Grenville rocks and/or input from northern sediment sources supplied the older Grenville-aged detrital zircon grains present in the Norphlet erg in the area to the west and within the Conecuh embayment.