Portal de Periódicos da CAPES

Life Orientation and Post-Mortem Reorientation of Chesterian Brachiopod Shells by Paleocurrents

1986; Society for Sedimentary Geology; Volume: 1; Issue: 3 Linguagem: Inglês

10.2307/3514693

1938-5323

Richard R. Alexander,

Geology and Paleoclimatology Research

Pediculate, biconvex brachiopods from the Chesterian (Late Mississippian) Chainman Formation of the Confusion Range, Westcentral Utah, lived with their commissures steeply inclined to the sediment surface. This inference is based on 1) commensal epizoans localized anteriorly on both valves, 2) abrasion equally well developed on the posterior of both valves, 3) preservation of specimens attached to skeletal clasts, and 4) flume experiments that reveal the advantages of the erect orientation for lateral inhalant flow between the valves. Experiments show that concavo-convex brachiopods (Linoproductus, Inflatia) are hydrodynamically most stable in their reclining life orientation. Winged taxa (Punctospirifer) and non-winged taxa (Anthracospirifer, Reticulariina) with broad interareas are next in order of stability in their life orientation. Brachiopods with a small interarea (Schizophoria) or no interarea (Rhipidomella) show moderate to very low stability, respectively. Brachiopods with curved hinge lines (Composita, Cleiothyridina, Eumetria, Hustedia) also show low stability in life orientation. Taphonomic data are consistent with these results. Concavo-convex and bioconvex, winged shells, as well as non-winged forms with broad interareas, were most often found anterior-posteriorly crushed by sediment compaction, indicating burial in life orientation. Forms with small interareas and with curved hingelines are often dorsoventrally flattened or laterally compressed, indicating reorientation by currents before burial and sediment compaction. This Chainman brachiopod fauna experienced moderate currents, competent to destabilize the shells selectively from the array of shapes. Frequencies of preservation of shells in life orientation are expected to vary with the hydrodynamic stability of the shape. This is a potentially reliable indicator of paleocurrent competencies in brachiopod community transitions across the paleoslope or from proximal to distal portions of tempestites.