Abstract: Lateral and Vertical Variations of Distal Turbidites in the Neoproterozoic Zerrissene Turbidite Complex, Namibia

Swart, Roger - NAMCOR

The well exposed Neoproterozoic Brak River Formation of the Zerrissene turbidite complex in north-western Namibia was deposited in a distal (outer-fan to fan-fringe) setting. The excellent outcrops in some areas approach a seismic scale of section and individual units can be followed for several kilometers. On a smaller scale excellent exposures of sedimentary structures are also found. The unit is characterized by sequences of Mutti and Ricci Lucchi facies B1, C2, D1, D2, G and an additional non-turbidite facies related to regional glaciation. Four facies associations have been identified and these represent lobe sandstone, fan-fringe, glacial marine/turbidite and basin plain facies. Finer-grained mud-rich units (basin plain mudstones and turbidites) separate packages of sand-rich units (lobe sandstones). It is probable that autogenic processes such as lateral shifting of facies controlled the switching from lobe sandstones to basin plain mudstones.

The complex appears to have been deposited adjacent to an ancient passive margin as:

- the preserved distal portion of the turbidite complex possibly covered an area of at least over 22,000 square kilometers before folding. This would make it one of the largest known ancient turbidite complexes.

- petrologically the turbidites are characterized by clasts of plutonic and metamorphic rocks and little volcaniclastic material.

- on a bed-by-bed scale no systematic trends have been recognized.

- the high sand:shale ratio found (often greater than 10:1) is similar to that which has been recently found in modern passive margin settings.

- interbedded laterally continuous carbonates suggest the presence of a stable margin during high stands.

The beds have been turned vertical during deformation, which means that individual beds can be followed on aerial photographs. Detailed photographs using a conventional 35-mm SLR camera were taken along some of the folds and in some cases individual beds can be followed for 7 km. A channel with a depth of 25 m and a width of 1025 m was also recognized. This feature is comparable to channels on the modern Mississippi Fan which have a depth-width ratio of 5:200. On the ground it is clear that this feature comprises a number of beds which indicates that it acted as a conduit for multiple flows and is not a scour feature resulting from a single event. A channel of this magnitude would be too small to pick out reliably on multi-channel seismics.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil