EXCEPTIONALLY PRESERVED SHELL BORINGS IN THE MIO-PLIOCENE ALEXANDRIA FORMATION, EASTERN CAPE, SOUTH AFRICA

Recent investigations of the Mio-Pliocene Alexandria Fm in the Eastern Cape of South Africa sheds light on the sedimentologic and paleontologic history of marginal marine environments in southern Africa. Sedimentary facies and abundant trace fossil represent an offshore to onshore transition. Detailed study of the Coega Brick quarry section, 20 km north of Port Elizabeth, reveals a suite of exceptionally preserved shell borings that provides insight into the taphonomic and diagenetic history of the Alexandria Fm.

We identified 10 units in the Alexandria Fm that record a progradational offshore to onshore transition. The seven associated facies represent middle shoreface, upper shoreface, foreshore, and backshore tidal flat and tidal channel environments and include: 1) fossiliferous polymict pebble-cobble conglomerate (shoreface revinement surface); 2) shelly calcarenite (shoreface); 3) sandy coquina (shoreface); 4) coarse sandy calcarenite (shoreface-foreshore transition); 5) biomicrudite (shoreface/foreshore beachrock?); 6) quartz arenite (backshore/tideflat); and 7) interlaminated thin cherty claystone and quartz sandstone (supratidal backshore/tideflat).

The biomicrudite facies contains an assemblage of shell boring trace fossils. Ichnotaxa include Entobia cateniformis, E. megastoma, Entobia spp., Caulostrepsis spiralis, C. spp., Talpina sp., and abundant microborings. This assemblage indicates a complex diagenetic history. Initial borings were made in skeletal substrates. Next, marine sediments filled borings. In some cases, the substrate/boring interface was cemented and subsequently filled with biogenic carbonate silt. Finally, diagenetic (meteoric?) fluids preferentially dissolved aragonitic host skeletons producing exceptionally preserved casts.

This trace fossil assemblage is the oldest Entobia ichnofacies yet reported from the southern hemisphere and as such advances understanding of the diversity and paleogeography of Cenozoic shell boring faunas. Furthermore, the sequence recorded in this assemblage highlights the potential utility of shell boring trace fossils for preserving complex taphonomic and diagenetic histories.