Abstract: Mississippian Subtidal Stromatolites in a High-Energy Shoal Setting, Salem Limestone, Indiana: An Analog for Modern Stromatolites

Brian D. Keith, Todd A. Thompson

Subtidal stromatolites are present within skeletal grainstones of the Salem Limestone (Valmeyeran, Mississippian) in south-central Indiana. These stromatolites are a close morphologic analog to modern subtidal columnar forms found in Shark Bay, Australia, and on the eastern Bahama Bank. The size, shape, morphology, internal structure, and depositional setting of the Salem stromatolites are closer to modern examples than to other Paleozoic or Precambrian stromatolites.

The stromatolites are composed of fine-grained skeletal debris and micritic laminae, and range from laminated crusts to large columnar forms up to 0.6 m high. They grew on hardgrounds and large lithoclasts present in the upper part of the Salem. Syringoporoid corals are also found in intimate association with some of the stromatolites.

Skeletal grainstones of the Salem formed in predominantly high-energy shoal complexes seaward of quieter lagoonal settings where muddier carbonate sediments accumulated. The stromatolite locality occurs along what was the seaward margin (ramp) of a major shoal complex. The lower 5 m of the Salem grainstone sequence is predominantly landward-oriented climbing trough cross-sets deposited during abundant sediment supply. Several undulatory hardgrounds are found within this sequence. The overlying sequence contains much thinner beds of bioturbated low-angle trough cross-sets with bidirectional indicators deposited under continued high-energy conditions, but with a more limited and intermittent sediment supply. Several undulatory hardgrounds of limited areal extent and one extensive planar hardground are present in this upper sequence. This latter surface shows evidence of induration to a depth of several centimeters, borings, rip-up clasts, and numerous overhangs. The majority of the stromatolites developed on this hardground and associated rip-up clasts.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994