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No abstract

A trace-fossil assemblage is exposed in the uppermost portion of an interpreted tidal sand-flat deposit in the Upper Mississippian Tar Springs Sandstone in southwestern Illinois. Sedimentary structures, indicative of a tidal origin for this unit, include herringbone crossbeds, flaser and lenticular beds, reactivation surfaces, small channel scours, and shrinkage cracks. The trace fossils include trackways, trails, feeding structures, dwelling burrows, and escape structures. Vertical dwelling and escape structures dominate the assemblage with lesser numbers of horizontal and inclined traces. The ichnological and sedimentological characteristics are diagnostic of the Skolithos ichnofacies. Preservation of the assemblage at the top of the Tar Springs Sandstone was the result of substrate stabilization as the tidal flat subsided and was transgressed. The present trace-fossil assemblage probably was formed by a combination of a pre-omission tidal-flat suite and an omission suite formed during and after the transgression. Preservation of the assemblage suggests a passive transgression with minimal scouring and reworking of the tidal-flat deposit.

FIG. 102. Growth surfaces of interior walls.--1,2. Metrarabdotos tenue (BUSK), ree., Caroline Sta. 68 off NE. Puerto Rico; 1, frontal exterior, growth surface, etched, note numerous superimposed layers accreting laterally simultaneously in a manner analogous to gastropod nacre, X8,000 (bar = I ~m); 2, detail of lower left region of 1, X 16,000 (bar = I ~m); both USNM 209434.--3.4. Tremogasterina robusta (HINCKS), rec., Perim Is., Aden, Red Sea; basal interiors, distal toward top of photograph; note rhombic crystal shapes, accretionary banding, and occasional screw dislocations; both X5,000 (bar = 2 ~m), BMNH 1966.2.24.1.--5. Arachnopusia unicornis (HUTTON), ree., N.Z.; basal interior; screw dislocations more common than in 3.4 and accretionary banding virtually absent; X2,000 (bar = 5 ~m),

Amsossiporo 20 F,.suggests that this genus may be a rrepostomate. It is questionably retained in the Ceramopotidae, whete it was placed by Y AROSHINSKAYA and ASTROVA.] U.Ord., USSR.--FIG. 156,2a,b. •A. simplex, paratype, SNIlGGIMS 951/523a, 523b; a, subcirculat to subangu1ar aurozooecia, acanthostyles at zooecial corners; tang. sec., X40; b, irregular thickness of aurozooecial walls, sparse diaphragms, acanthostyles associated with budding loci; long. sec., X20 (photographs courtesy of G. G. ASTROVA).