Seismic-stratigraphic models for late Pleistocene/Holocene incised valley systems on the Durban continental shelf.

Abstract

This dissertation examines the Durban continental shelf of the east coast of South Africa from a

seismic and sequence stratigraphic perspective. High resolution seismic data reveal eleven

seismic Units (A-K) offshore the Durban continental shelf comprising several partially preserved

sequences. Unit A is the lower most unit, comprising Permian age shale of the Pietermaritzburg

Formation. An early Santonian age is assigned to Unit B. The ages of Units C and D are

indeterminate. Unit E is considered late Maastrichtian in age. Units F to I are assigned a late

Pliocene age and represent an aggradational progradational shelf-edge wedge. Unit J comprises

calcite cemented late Pleistocene/Holocene shoreline deposits which display morphologies

similar to planform equilibrium shorelines on modern coasts. Unit K caps the stratigraphy and

comprises a seaward thinning, shore-attached wedge of Holocene age. The lower portions of

Unit K comprise the fills of an extensive LGM age incised valley network.

A widespread network of incised valley systems on the continental shelf offshore Durban was

recognised and examined; the evolution of which were compared over time. These incised

valleys represent the shelf extension of the main river systems in the area, namely, the Mgeni,

Mhlanga and Mdloti rivers as well as those that drain into the Durban Harbour complex. In the

study area late Pleistocene/Holocene aged valleys occur together with a subsidiary series of late

Pliocene isolated valleys. Valleys of the last glacial maximum (LGM) of ~ 18 Ka BP exhibit

simple fills and have intersected and reworked or completely exhumed the late Pliocene incised

valleys. Only isolated examples of these late underlying Pliocene valleys are apparent.

Twenty five prominent incised valleys are recognised within the study area and occur

predominantly in the mid-outer shelf. These valleys mainly incise into Cretaceous age rock,

except for a few incisions occurring within Permian age shale of the Pietermaritzburg Formation.

Six seismic units (Units 1-6) comprise the infill material within the late Pleistocene/Holocene

incised valleys, and on the basis of their architecture are interpreted to correspond with a

succession from high energy basal fluvial deposits, low-energy central basin fines, mixed-energy

estuarine mouth plug deposits, clay-rich flood deposits through to capping sandy shoreface

deposits. The LGM aged fills in particular have volumetrically thick fluvial deposits, the result of

increased gradient and stream competence during the LGM. The youngest valleys show a

situation of differential evolution along the valley length due to varying rates of sea level rise in

the Holocene. Initially, rapid sea level rise caused drowning and overstepping of the outer

segment of the incised valley. During the late Holocene, slower rates of sea level rise caused

shoreface ravinement of the inner-mid segments of the valley and created an imbalance between

accommodation space and sediment supply, producing different facies architectures in the

valleys. This differential exposure to accommodation has resulted in a sedimentological

partitioning between tide-dominated facies in the outer valley segment and river dominated

facies in the inner segment.

Due to significantly wider exposed coastal plain during lowstand intervals, the rivers in the study

area avulsed and coalesced on this lowstand surface and thus possess no defined drainage

patterns. A crenulate shaped subsurface knickpoint occurs at a depth of ~ 50 m, and is considered

to have formed by initial slow ravinement processes that graded the antecedent shelf, followed

by overstepping and preservation of the knickpoint during meltwater pulse 1B.