CARBONATE FACIES, DEPOSITIONAL SEQUENCES AND TECTONOSTRATIGRAPHY OF THE PALAEOGENE MALTA PLATFORM

Abstract

The break-up of Pangaea and the Late Mesozoic global sea-level rise drowned many Tethyan carbonate platforms although the resilient Malta Platform aggraded >4 km of carbonates along the North African passive margin where it was isolated from continental siliciclastics. Carbonate sedimentation was terminated by extensive Late Cretaceous to Early Paleogene depositional hiatuses, but renewed during the Oligocene, when basinward carbonate progradation began to drape over the >350 km long, cusp-shaped escarpment along the eastern margin of the isolated platform.

This study sub-divides the Oligocene sediments of Malta into eight facies associations. The facies consist of carbonate grains of coral, coralline red algae and large benthic foraminifera which dominated sediments of the Late Rupelian to early Chattian, mid-Chattian and late Chattian, respectively. These successive carbonate factories produced the photozoan-heterozoan-photozoan triplet of carbonate grain associations which, when dated by benthic foraminiferal biozonation, correlates to the succession of carbonate grain associations in other Mediterranean carbonate platforms.

The sedimentary triplet reflects abrupt changes in carbonate ecosystems that coincide with the last three of six surfaces that extend >80 km around Malta. The surfaces show evidence of the influence of meteoric water and pedogenic processes recognised by diagenetic features and isotopic excursions. These sequence boundaries sub-divide the succession into seven depositional sequences that reflect global third-order cyclic sea-level falls produced by glaciations with a periodicity of 1.2 Ma triggered by low-amplitude obliquity variations of the Earth’s axis combined with orbital eccentricity cycles.

The periodic growth of the Antarctic ice-sheet during the Oligocene also affected Tethyan climate by shifting low latitude climate belts northwards. It is suggested that increased aridity over North Africa had reduced nutrient flux to the Tethys and favoured photozoan carbonate biota over the Malta Platform and other Tethyan carbonate platforms. The stepwise decrease in oxygen isotope ratio by the mid-Chattian reflects Antarctic deglaciation that increased both precipitation over North Africa and nutrient flux in the Tethys, favouring heterozoan ecosystems.

The mid-Chattian transgressive heterozoan carbonates draped over structured bathymetry of an antecedent extensional regime that produced rotated fault-blocks. Highstand shedding of coralline red algae resulted in large clinoforms prograding into partly filled NNE trending half-graben (<10 km-wide) in the Maltese Islands whereas block rotation involving deep, en echelon listric faults formed escarpments along the platform margin. The escarpments were initially onlapped by syntectonic early Palaeogene sediments and later downlapped by prograding complexes. The central platform zone developed as a >50 km-wide basin by lithospheric sagging over a failed Mesozoic rift.

The late Chattian climatic optimum was reflected by a further decrease in the oxygen isotope ratio and aridity over North Africa and favoured a return to the photozoan association during the last phase of the Oligocene sedimentary triplet. Lepidocyclinids flourished in inner to mid-platform environments forming banks although the rate of accumulation of these hydrodynamic foraminifera did not keep up with sea-level rise. The shift to increased trophic resources by the end Oligocene terminated shallow marine carbonate sedimentation which resulted in the drowning of the Malta Platform.