--> Late Silurian-Early Devonian Paleogeography Part 1 - Basal Sands of the Sealevel Lowstand

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Late Silurian-Early Devonian Paleogeography Part 1 - Basal Sands of the Sealevel Lowstand

Abstract

During Pridoli and Early Devonian, an exceptionally large magnitude, long lasting sealevel lowstand exposed huge areas of the North American craton that had formerly been sites of marine deposition. For millions of years, weathering and erosion became dominant geological processes. Karst was well developed within widespread carbonates, and a major unconformity developed. Sands were produced by erosion of highlands. With removal of fines (sedimentary bypassing) and carbonates, additional sands accumulated by concentration. During Early Devonian, most of the craton was exposed, and sands continued to accumulate in structurally higher settings through Middle and into Late Devonian. Middle and Late Devonian sealevel fluctuations were dominated by a progressive sealevel rise. As sealevel rose, unconsolidated sands that had accumulated primarily during Early Devonian were incorporated as basal beds of many different deposits. Traditionally, we have dated these sand intervals to match the ages of overlying deposits, but we should not allow that to blind us to the fact that they originated primarily during Early Devonian. In moderately low/deep portions of basins, exposed as dry land during the Early Devonian sealevel lowstand but subsequently flooded by Middle Devonian sealevel rise, sands are incorporated at and near the base of Middle Devonian carbonates. Sands continued to accumulate in terrestrial environments in areas not yet inundated by rising Middle Devonian seas. These were later incorporated at the base of Upper Devonian/lower Mississippian black shales. We regard these sands as ‘younger’ because areas in which they occur were not inundated until this much later period. However, the ‘younger’ sands beneath the black shales accumulated throughout the same exposure period as the ‘older’ sands at the base of Middle Devonian carbonates. All of these sands are time transgressive in terms of when various portions of the sand bodies were inundated and incorporated into sediments later lithified. Within individual stratigraphic sections, it is possible, even likely, to have sands occur at multiple levels. In areas previously inundated, with the principal sand at the base of the oldest post-Early Devonian rock unit, blankets of windblown or waterborne sands might subsequently be transported from higher areas not yet inundated.