| Summary |
The Yazoo Clay in eastern Mississippi and southwestern Alabama has four members that grade westward into an undifferentiated clay over 400 ft (121.9 m) thick. This unusually thick section of the Yazoo Clay at Mossy Grove contains a largely uninterrupted record of late Eocene to early Oligoc̈ne geologic and biologic events. Although the Yazoo Clay is not organic-rich by traditional standards, the nitrogen content varies directly with benthic faunal morphotypes that prefer organic-rich and dysoxic conditions. Faunal and sedimentological characters indicate that such conditions prevailed throughout the depositional history of the Yazoo Clay. During the time that the Yazoo Clay was being deposited, there were no major changes in sedimentological, geochemical, or faunal parameters except in the interval beginning at core depth 176 ft (53.6 m). At and immediately above this level, there is a large influx of silt, high total carbon values reflecting lag deposits of molluscan debris, a chaotic mix of benthic foraminiferal faunas represented by a series of single sample clusters of both genera and morphotypes, and negative excursions in both total organic carbon and nitrogen. Combined, these data strongly suggest an "event" (possibly an unconformity) at this level in the core. Unconformities at the base and top of the formation indicate mixing with the underlying or overlying sediment. Overall, the deposition of the Yazoo Clay occurred in a relatively stable environment with only gradual changes in sea level, organic matter and oxygen levels. The general depositional environment was an outer neritic to upper bathyal setting under deltaic influence. |
