Structural and stratigraphic constraints on the relationship between extension and landscape evolution in the Early Mesozoic Eastern Gulf of Mexico

Abstract

Landscape of the early Mesozoic intracratonic Eastern Gulf of Mexico (EGOM) was characterized by several parallel high-relief (~5 km) topographic ranges, composed of rocks of the Allegheny orogen and Suwannee terrane, which exerted strong influence over early Mesozoic EGOM depositional patterns. Varying early Mesozoic rift fault geometries and displacement, along with topographic relief partly controlled by differential erosion of lithologic and structural heterogeneities inherent in basement rocks, imparts significant consequences for sediment transport pathways. Detrital zircon provenance of Mesozoic units in the northern EGOM region of onshore southern AL and FL panhandle region indicates moderately well integrated, regional drainages tapped a broad catchement that included Alleghany foreland strata, Alleghany orogenic rocks, and peri-Gondwanan accreted terranes of the Pangean upper plate. In contrast, south of the Middle Ground arch/Southern platform along western Florida and adjacent offshore areas, brittle upper crustal extension formed more broadly spaced uplifts, and sedimentary provenance indicates predominance of small, highly localized drainages, with limited sediment influx derived primarily from erosion of local Suwannee terrane rocks of the Pangean lower plate. The differences in sediment transport and rift structures between northern and southern EGOM is attributed stronger rheologies of Paleozoic and older cratonic lithosphere of the Suwannee terrane that formed the upper plate during Alleghanian collisional tectonic compared to the weaker lower plate continental lithosphere of the peri-Gondwanan terranes of southern AL and FL panhandle. Differences in rheology may result from different thermal histories during latest Paleozoic Alleghanian orogenesis. As ductile lithospheric thinning continued after the cessation of brittle crustal rifting at ~180 Ma, under-filled EGOM early Mesozoic topographic lows continued to influence depositional patterns observed in younger Mesozoic units. Introduction & Background The early Mesozoic EGOM basement was modified by rift faulting, forming the on-shore and offshore rift basins which influenced sedimentation patters during early EGOM evolution. (Chowns and Williams, 1983; Klitgord et al., 1988; Withjack et al., 1998; Schlische et al., 2003; Heffner, 2013). However, southward along the EGOM margin, early Mesozoic uplift geometry become broader and are separated by wider rift basins (Fig. 1). Early Mesozoic rift basins of the northern EGOM were developed in lower plate continental lithosphere of the Alleghany orogen, accreted peri-Gondwanan terranes, whereas southern EGOM rift basins developed in upper plate continental lithosphere of the Gondwanan Suwannee terrane, which is separated from lower plate rocks by the Suwannee suture (Fig. 1; Williams and Hatcher, 1982; Mueller et al., 2014). Distinct thermal histories of lower plate versus upper plate rocks during latest Paleozoic orogenesis likely contributed to different strain response to extension in the Early Mesozoic. To investigate the varying landscapes and drainage patterns as a proxy for crustal response to extension along the eastern EGOM margin, we leverage the distinct geologic histories of rocks suites in EGOM lithospheric units to apply detrital zircon U-Pb provenance in reconstructing the topographic, erosional and structural evolution of the early Mesozoic EGOM region. Methods Detrital zircon provenance analysis of Early Mesozoic clastics collected from core and drill cuttings from wells along the eastern margin of the EGOM were subjected to LAICPMS U-Pb zircon analysis following the methods of Gehrels et al. (2014). We have analyzed 4 samples from the South Georgia rift basin, 2 samples from the Talahassee graben and 3 samples from the South Florida basin and 2 other basal clastic samples from central Florida. We compare these results to our previous detrital zircon provenance analysis of over 2,200 zircon ages from 26 samples of the Norphlet Formation from Mississippi, Alabama, Florida and offshore federal waters, as well as 3 samples of Haynesville clastics from Alabama and Florida. Zircon provenance results are combined with interpretation of basement structures from 2D seismic profiles of the Florida outer continental shelf to examine the relationships between early Mesozoic landscape, sediment transport patterns and structural controls on rift basin development. Results & Discussion In onshore southern AL and western FL panhandle, several topographic ridges, which are features related to the Alleghanian orogeny. These ridges controlled early Mesozoic deposition in the up-dip areas of the northern EGOM. To the east, the South Georgia rift system and its southern extension of the Tallahassee Graben, includes several half-grabens bound by listric faults (Chowns and Williams, 1983; Heffner, 2013). Analysis of 2D seismic profiles along the EGOM margin are used to relate onshore extensional basement structures to offshore Mesozoic rifting. A continuous curvilinear magnetic anomaly low links the South Georgia Rift/Tallahasee Graben with the Apalachicola Basin half-graben, indicating these were part of one larger early Mesozoic graben system. Sedimentary provenance of South Georgia rift samples exhibit a mix of detrital zircon ages, but nearly 60% of ages range between 250-525 Ma indicating input from early Alleghanian orogenic rocks, along with zircon from Neoacadian orogenic rocks and Taconic orogenic rocks likely recycled from sedimentary cover strata. The 525-740 Ma population indicates some input from Suwannee terrane and a 920-1,175 Ma population indicates input from erosion of Grenville rocks or recycling of Grenville zircon from sedimentary cover strata. Down depositional dip, the Tallahasee Graben (TG) exhibits an even greater diversity of zircon ages than the SGR samples. Ages similarly indicate input from Alleghanian, Acadian and Neoacadian orogenic rocks or recycling of sedimentary rocks that contain zircons from these orogens, as well a greater abundance of 500-710 Ma and 2,080-2,200 Ma zircon from the Suwannee terrane and 1,000-1,270 zircon from Grenville rocks or recycled Grenville zircon from sedimentary cover strata. We interpret this downstream increase in complexity to reflect contributions of various zircon age populations from downstream tributaries. Toward the southern EGOM, the NE-SW Tampa Embayment-West Florida basin is separated from the Appalachiacola Basin to the north by the Middle Ground arch and from the South Florida basin by the Sarasota arch. Basement structures indicate contemporaneous northwest-southeast Late Triassic extension, however subsidence diminishes toward the south, preventing deposition of salt that is found in the northern EGOM. Five samples of basal clastics of the Wood River Formation or equivalent deposits, which likely record early syn-rift sedimentation, were analyzed updip from the Tampa Embayment and from within the South Florida basin. In contrast to the diversity of detrital zircon ages observed in syn-rift deposits of northern FL and southern GA, the SFB clastics display a striking lack of diversity and instead are dominated by a 550-650 Ma population with a major peak at 605 Ma and a minor population from 2.0-2.2 Ga, consistent with derivation from Suwannee terrane rocks. This matches other ages for the Suwannee basement rocks as well as our zircon ages of ~550-650 Ma from phyllitic basement rocks penetrated in ODP 77 in the Yucatan Channel of the southeastern GOM. No input from upper plate sources is detected anywhere south of the Talahassee graben in our work. Instead all zircon ages reflect derivation from mainly Neoproterozoic basement rocks and associated early Paleozoic sedimentary cover of the Suwannee terrane. Similar signatures are observed in detrital zircon ages is also observed in samples of the Upper Jurassic Norphlet and Haynesville Formations (Weislogel et al., 2014; Essex et al., 2016; Hunt et al., 118 2017). A mix of Gondwanan and Grenville zircon were shed into the Norphlet erg of southern Alabama and the Florida panhandle, indicating sediment feeder systems contained a mixture of detritus from upper plate and lower plate source areas. Detrital zircon signature of the Norphlet and Haynesville Formation samples indicates an integrated drainage did not extend past the Tallahassee graben. South of the Tallahassee graben, these Upper Jurassic units also reflect contribution of primarily Neoproterozoic zircon derived from Suwannee terrane rocks. Together these data indicate crustal rheology that impacted Triassic extensional strain controlled the geometry of rift basins and faults, which in turn impacted the nature of sediment transport systems that fed sediment into the basins; these impacts persisted through deposition of the younger Norphlet and Haynesville formations (Weislogel et al., 2015; Essex et al., 2016).

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