Sedimentation, sea-level change, and tectonics on an early Pleistocene continental shelf, northern California

The continental shelf of the Palaeo-Agulhas Plain (PAP) is scattered with Pleistocene deposits with subdued topography. Their exaggerated lateral extension is the expression of a flat underlying substrate and availability of accommodation space, depositional processes and response to glacio-eustatic sea-level change have influenced deposition and distribution of these units. We present new results for the upper ~30 m (up to ~200 ka) of the stratigraphic record in this area and show that this shelf offers the opportunity to examine the response of a stable tectonic setting to the effects of sea-level change. This paper presents the results of extensive sub-bottom profiling surveys and chronostratigraphic investigations from marine sediment vibracores. Radiocarbon and Optically stimulated Luminescence dates are integrated into a seismic stratigraphic model composed of twenty Quaternary units, where two depositional sequences are bounded by shelf-wide unconformities. The upper sequence was cored where Pleistocene deposits were observed to be close to the seafloor and are draped in a thin veneer of marine shelf sediment and allow us to describe the environments of deposition of the PAP. The most pervasive stratigraphic pattern in these shelf deposits is made up of the depositional sequence remnant of the Falling Stage Systems Tract (FSST) forced regression from Marine Isotope Stage 5e–2. The other dominant stratigraphic group is the Transgressive Systems Tract (TST) associated with the Postglacial Marine Transgression. Surprisingly, the TST makes up an almost equal proportion of deposits in both sequences in the sedimentological record as the FSST, despite the shorter temporal span of the TST. The sub-bottom profiles were acquired on regional surveys extending from the Breede River in the west to Plettenberg Bay in the east, and to a maximum depth of 110 m below Mean Sea Level, with the exception of one ~200 m deep shelf-edge profile.

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Indicatives of ancient conditions of sea level stability on the southern Pernambuco continental shelf

Pesquisas em Geociências ◽

10.22456/1807-9806.20266 ◽

2001 ◽

Vol 28 (2) ◽

pp. 25 ◽

Cited By ~ 2

Author(s):

MAXIMILIANO MICHELLI ◽

TEREZA ARAÚJO ◽

MAURO MAIDA ◽

HELENICE VITAL

Keyword(s):

Continental Shelf ◽

Sea Level ◽

Sea Level Change ◽

Main Interest ◽

Level Change ◽

Meandering Channels ◽

Northeastern Shelf ◽

Brazilian Northeast ◽

River Mouths

The continental shelf in southern Pernambuco State (Brazilian Northeast) is characteristically narrow, and breaks around 32km from the actual coastline, in depths of 50 to 60m. The sediments on this shelf are similar in their constitution to the rest of the Brazilian Northeastern shelf, showing a prevailing fraction of coarse carbonatic sands. There is also a significant input of terrigenous material, which may be found mainly up to 20m deep. Regarding it’s geomorphology, Pernambuco’s southern shelf presents a soft slope, with long and smooth surfaces, sometimes crossed by meandering channels which show more frequently next to river mouths. Indicatives of sea level change like the presence of beach rocks and paleochannels occur along the shelf. The main interest of the present work lies on the location and characterization of the submerged beachrocks on the southern portion of Pernambuco State continental shelf. The area was investigated using sonographic techniques. Data analysis allowed to identify indicatives of ancient conditions of sea level stability on the shelf. The first evidence was found at a depth of 20m, where a beachrock of a triangular shape and rounded top with 136m wide and 6m height, was identified. The second evidence was registered at 40 to 50m depth also represented by a beachrock. Geomorphologically this second feature presents a relatively flat top, 209m of width and 4m height. Another structure indicative of sea level change identified was a paleochannel, which started at 30m depth and followed the direction northwest-southeast on the shelf.

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