ESTUARINE ICHNOLOGY IN THE MISSISSIPPIAN HARTSELLE SANDSTONE OF ALABAMA

The trace fossils of the Hartselle Sandstone were described in 1994, when few examples of estuarine ichnofacies had yet been published. Reevaluation of these specimens confirms that the Hartselle Sandstone at Fielder Ridge (Colbert County) was deposited in estuarine conditions. The formation as a whole is highly diverse and includes marine to terrestrial deposits.

The section at Fielder Ridge is transgressive and can be divided informally into three stratigraphic units. The lower contact with the shale of the Pride Mountain Formation is unconformable. (1) The basal unit, consisting of tangentially crossbedded sandstone, is relatively poorly exposed and has yielded no fossils. (2) The middle unit is made up of thinly bedded, current-rippled sandstone with intercalated shale, yielding abundant trace fossils and scattered invertebrates. Conditions evidently ranged from marine to brackish; trace fossils of marine organisms (e.g., asteroid traces, Asteriacites stelliforme) occur alongside “marine” indicators that can now be recognized as ambiguously brackish or marine (e.g., arthropod traces Nereites missouriensis and “Rusophycus” hartselleanus). Brackish substrates were occupied by bivalves, crustaceans, and probable polychaetes, the last being represented by the dendritically branched Hartsellea sursumramosa. Extremely dense populations of Lockeia siliquaria show that bivalves utilized estuaries as nurseries as early as the Mississippian. (3) The uppermost unit, consisting of amalgamated, crossbedded, very fossiliferous calcareous sandstone with a basal intraclastic conglomerate, includes fragmented crinoids, blastoids, brachiopods, fenestellids, trilobites, and bivalves, with only a few trace fossils including Pentichnus; it is interpreted as a tidal inlet fill. The upper contact with the Bangor Limestone, here represented by marine calcareous shale, is conformable.

Since 1994, the distinctive trace fossils Hartsellea sursumramosa and “Rusophycus” hartselleanus have been reported only from sites in the Hartselle Sandstone. Further study of estuarine Mississippian deposits would likely yield new examples.