TESTING THE HEIGHT AND EXTENT OF THE LOWER ORDOVICIAN (SAUK) TRANSGRESSION USING THE ROCK RECORD ACROSS THREE CONTINENTS

The Sauk sequence (Cambrian-Lower Ordovician) is thought to have been deposited during one of the highest sea level episodes of the Phanerozoic. Although many authors have concentrated on detailing sea level changes during the Mesozoic, fewer have critically assessed the record of the Paleozoic. This paper examines the complete sedimentary record across North America, South America and Africa, sequence-by-sequence, in order to test the published sea level curves against the global rock record. Conclusions are drawn based on the actual rocks in place. This study gives particular attention to the presumed sea level maximum in the Lower Ordovician (Sauk sequence).

Stratigraphic columns across the three continents were compiled from published reports, outcrop data, oil wells, cross-sections and/or seismic data. We constructed 710 stratigraphic columns across North America, 429 across Africa, and 405 across South and Central America. Detailed stratigraphic and lithologic data were input into RockWorks 17, a commercial software program. Resultant models allowed accurate determinations of volume, extent and thicknesses for all six Sloss-defined sequences.

The continuity of the basal Sauk sandstone layer across North America is a testimony to the extent and uniformity of the first great marine transgression of the Phanerozoic. In many places, the base of this layer is also known as the Great Unconformity. Although the areal extent of the Sauk sequence across North America is quite impressive, it has the least volume of sedimentary rock preserved, compared to all subsequent sequences, and is thin or missing completely across the central part of the continental USA. In addition, Africa and South America exhibit very little evidence of massive continental flooding during deposition of the Lower Ordovician (Sauk sequence). Sauk sediments are only found in the northernmost part of Africa (beneath the Sahara and the Middle East) and the west-central portion of South America. In fact, the total volume of Sauk preserved was consistently one of the least across all three of the continents in this study. We conclude that previously published sea level curves are poorly defined for the early Paleozoic and that the Sauk sequence represented only limited flooding of the Gondwana landmass.