Sedimentations-Geschichte der Bermuda North Lagoon im Holozän (Sedimentation history of the Bermuda North Lagoon during the Holocene)

Gerhard.Kuhn [ at ]


Summary Holocene sediments of the North Lagoon, Bermuda, were studied with shallow seismic reflection profiles (200 km CSP-survey, UNIBOOM-system) and vibration coring (40 sediment cores, pneumatic vibration corer, Meischner et al., 1981). Seismic Stratigraphy Four seismic sequences are distinguishable by seismic stratigraphy. All seismic sequences correspond to depositional sequences built up during high sea levels in interglacial times. The seismic sequences are separated by unconformities which are often strongly reflective and correspond to emersion planes during glacial phases. The upper sequence (sequence 4) is related to Holocene sediments. The pre-Holocene bedrock is divided into three different seismic sequences (Kuhn et al., 1981): Sequence 1: oldest Pleistocene sequence (pre-Sangamon sea-level highstands), upper boundary with levelled relief (lower boundary not discernible), composed of strongly cemented carbonate sediments, forms the bedrock below Three Hill Shoals Sequence 2: Sangamon (125 ky sea-level highstand), distinct surface morphology, forms the bedrock of a large area below Holocene sediments, Holocene reefs grew up on elevations of the sequence 2 surface, the Holocene reef rim was developed on an elevated rim of sequence 2 Sequence 3: youngest Pleistocene sequence (Sangamon, 105 and 85 ky sealevel highstands lower than recent), deposited mainly in depressions of the bedrock deeper than -15 m below recent Mean Sea Level, levelling the older relief, peat sedimentation in places The distribution of recent reef areas and lagoonal basins is strongly controlled by pre-Holocene topography and geology of the bedrock. During the Holocene approx. 1050 x 106 m3 of carbonate sediments were deposited in the North Lagoon (290 km2) and approx. 1350 x 106 m3 in the reef rim area (170 km2). Sedimentology There are no larger oscillations of the Holocene sea level identifiable in the sedimentological record. The pre-Holocene topography was gradually drowned during the Holocene sea-level rise. At first, the depositional depressions were separated and landlocked. Fresh water peat marshes, fresh water ponds, marine ponds and bays were formed. With rising sea level, the land barriers were more and more eroded, drowned and lost their influence on the back-barrier sedimentation area. Autochthonous and allochthonous peat, lime gyttja and carbonate mud are a typical transgressive back-barrier sediment sequence. After destruction of the barrier, the depositional milieu changed from restricted marine to normal marine, open lagoonal. Sea-grass sediments and nearly mud-free carbonate sand were deposited in shallow water in an exposed environment. Hydrodynamic energy decreases with increasing water depth in the lagoonal basin. A more densely growing reef rim and intralagoonal reef growth added to the protection of the deeper lagoonal floors. Fine-grained sediments were deposited in this environment. They are distributed over a large area of the North Lagoon and form the top of the transgressive lagoonal sediment sequence. Holocene reefs mainly developed on rises of the pre-Holocene surface. In the early Holocene, solid reef build-ups were able to keep up with the rapid rise of sea level. Sand pockets in the reefs were left behind and filled up mainly in the later Holocene. The percentage of fine-grained sediments, produced and resuspended in the reef rim and deposited in the near lagoonal back-reef zone, increased during the Holocene. Two models of Holocene sedimentation in a depression and on an elevation of the pre-Holocene surface illustrate the dependence of vertical facies gradation on pre-Holocene topography. Trends of the mostly polymodal grain-size distributions of the Holocene sediments are a coarsening-upward in the back-barrier and a fining-upward in the lagoonal sediment sequences. Change in the composition of the molluscan fauna in the Holocene sediments (particle size > 2000 µm) is an Indication for fades changes. Gastropods are abundant in the basal backbarrier sediments. Bivalves are rare and their diversity 1s low. Sea-grass sediments contain Codakia orbicularis and Astraea phoebia shells. In the sheltered lagoonal environment shell fragments > 2000 µm become rare, common species are Gouldia cerina, Pitar fulminata and Finella sp. (approx. 1000 µm). Fine-grained reef-rim derived sediments differ from lagoonal sediments by a higher percentage of Homotrema rubrum fragments and Alcyonaria spicules.

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Kuhn, G. (1984): Sedimentations-Geschichte der Bermuda North Lagoon im Holozän (Sedimentation history of the Bermuda North Lagoon during the Holocene) , PhD thesis, Geologisch-Paläontologisches Institut der Universität Göttingen.

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