Abstract: Parasequence-Bounding Sandy Event Beds in the Upper Cretaceous, Alabama: Their Origin, Periodicity, and Hydrogeologic Significance
David T. King, Jr., Michael C. Skotnicki
In two separate, 20-m thick intervals within the upper part of the Blufftown Formation (lower Campanian, 82-80 Ma), sandy event beds delineate the boundaries between thin inner-shelfal
parasequences. The two 20-m intervals studied comprise the uppermost parts of highstand systems tracts within two separate and much thicker Blufftown depositional sequences. Individual inner-shelf parasequences range in thickness from 1.6 to 2.2 m and are comprised of a basal sandy event bed and an overlying layer of inner-shelfal glauconitic clayey silt or silty clay. The basal sandy event beds (averaging 0.5 m thick) rest upon scour surfaces and are correlative up to 25 km in outcrop and shallow subsurface. The sandy event beds display a gradual seaward progression of textures varying from (1) distal lower shoreface sandy conglomerate containing abundant Exogyra valves to (2) inner-shelf coquinoid graded sand, and ultimately to (3) hummocky cross-stratified silty sand. Thes event beds are interpreted to have formed by single or multiple episodes of suspension by storm-wave driven, cyclic wave loading and associated liquefaction of shoreface and nearshore clastics. The suspended clastics subsequently flowed seaward as low-slope turbidites that were later modified by deep-acting storm waves. Estimates of average periodicity for the sandy event beds are 33 and 44 ka, respectively, for the lower and upper 20-m intervals studied. Milankovitch-obliquity cycles, averaging 41 ka duration, may account for the climatic and sedimentologic conditions necessary for genesis and, more importantly, preservation of the sandy event beds in both intervals. The 20-m intervals are local minor artesian aquifers within the Blufftown aquifer, especially in areas where fractures c t through the sandy event beds. In some areas, the sandy event beds, possessing permeabilities of up to 1.1 darcys, are significant contributors to the total downdip leakance from coeval barrier-island and lower shoreface aquifer facies within the Blufftown aquifer system.
AAPG Search and Discovery Article #90983©1994 GCAGS and Gulf Coast SEPM 44th Annual Meeting, Austin, Texas, October 6-7, 1994