INCISED VALLEYS, TREPTICHNUS PEDUM, AND THE PRECAMBRIAN-CAMBRIAN BOUNDARY IN NAMIBIA

The Nama Group, southern Namibia, has well developed incised valleys which incorporate the Precambrian-Cambrian boundary. Previous work showed that incised strata (Spitskopf Member) include ash beds dated at ca. 543 Ma and contain the Ediacaran fossils Pteridimium and Swartpuntia. Strata (Nomtsas Formation) that infill the incised valleys contain an ash bed dated at ca. 539 Ma and also contain abundant trace fossils, including abundant Treptichnus pedum whose first appearance marks the base of the Cambrian. These data suggest an unconformity with a duration of up to ca. 4 Myr.

Recent mapping shows that strata of the Spitskopf Member experienced at least two episodes of incision and that the Nomstas Formation is a compound unit of superimposed incised valley fill strata. The stratigraphically older incised valleys cut deepest, and are backfilled by strata that locally exceed 100 m in thickness. Valleys are only locally developed. Facies are largely deeper water (sub-wave base) finely laminated silstones and shales that contain sparse interbedded medium-grained turbiditic sandstones. The valley floor is characteristically marked by coarse, carbonate-clast conglomerate derived from erosion of the underlying Spitskopf Member. Conglomerates were emplaced as debris flows and are poorly organized. The 539 ash bed occurs at the base of this older incised valley succession; however, T. pedum is absent in these deeper water facies.

The younger incised valleys cut the fill of the older incised valleys, but are shallower (less than 40 m) and extend more broadly across the basin. These too have a basal deposit of coarse, carbonate-clast conglomerate, which is overlain by laminated siltstones and shales. These in turn pass upward into a coarsening-upward succession of fine to medium-grained sandstones. These sandstones are laminated in cross-section, but in plan view may be completely saturated with traces of T. pedum. The degree of bioturbation is closely linked to primary facies: laminated fine sandstones are favorable, particularly where interstratified with thin interbeds of fine-grained, cross-bedded sandstones. These sandstones show variable, sometimes bi-polar paleocurrent directions suggesting tidal influence. These observations suggest a strong environmental control on the distribution of T. pedum.