NEOPROTEROZOIC PROVENANCE OF THE EARLY JURASSIC NORPHLET FORMATION, MOBILE BAY, ALABAMA: MEXICAN AND SOUTH AMERICAN CONNECTION WITH EASTERN GULF OF MEXICO THROUGH CONTINENTAL SCALE PALEO DRAINAGE SYSTEM

Detrital zircon geochronology established unroofing of Laurentian, Gondwanan and peri-Gondwanan rocks in the relict Alleghany orogen contributed the dominant clastic influx to the Eastern Gulf of Mexico (EGOM) during Jurassic Oxfordian time. Recently, additional work has identified potential Mexican and/or South American sources for the Jurassic Norphlet Formation sediments in the EGOM.

Five samples have been collected over the lower 88 ft. of the Norphlet Formation from a single borehole core in Mobile Bay, Alabama. Detrital zircon U-Pb ages determined by LA-ICPMS reveal that the upper four samples (22,002 -22,073 ft.) are characterized by abundant Grenville (950 – 1250 Ma) ages with minor peaks at 360 Ma, 420 Ma, and 460 Ma, likely derived from Acadian and Taconic orogenies. A Neoproterozoic age population (850-920 Ma) constitutes 30% of the lowermost sample collected at 22,089 ft. Potential sources of these zircons include 1) recycling Paleozoic sedimentary rocks of the Mixteca terrain of central Mexico and 2) the Neoproterozoic Goiás magmatic arc of Brasilian Belt in west-central South America. Analysis of Hf isotopes for 850-920 Ma detrital zircon grains from the Norphlet show εHf_(t) values range from +10 to -5, with most values ranging between +5 to -3. Other studies have shown εHf_(t) values of +1 to -4 for 850-920 Ma zircon from the Mixteca terrane and +2 to +12 εHf_(t) values for similar age zircon from the Goiás magmatic arc.

Since there is overlap, it is possible that rocks from either or both the Mixteca terrane or Goais arc are potential sources. Additional work is needed to address the definitive origin of 850-920 Ma detrital grains; however, the presence of Neoproterozoic zircons in EGOM support a paleogeographic model where sediments delivered to the EGOM through eastern Mexico and/or northern South America during Jurassic time. We hypothesize that a rift between North America and rotating Yucatan block may have acted as a conduit for sediment dispersal from western and southern sources, terminated during continental rifting leading to oceanic sea-floor spreading and encroachment by Pacific Ocean water.