Climatic controls on facies in Palaeogene Mediterranean subtropical carbonate platforms

Abstract

Cenozoic carbonate platforms in the central Mediterranean region show distinct vertical changes in carbonate skeletal assemblages and porosity characteristics that reflect shifts in environmental conditions affecting the western Tethys. The Photozoan Association produced by carbonate ecosystems adapted to low nutrient environments and the Heterozoan Association favoured by mesotrophic conditions alternate through time over the Malta Platform and nearby carbonate platforms, although not in phase with trans-Mediterranean Oligocene carbonates. This anomaly reflects the transitional nature of Cenozoic climate as well as continental convergence of the Tethyan margins. Restricted conditions amplified the effect of nutrient flux from North African fluvial systems, which was controlled by meridional shifts in the inter-tropical convergence zone (ITCZ) precipitation belt. We model the climate-carbonate interaction by comparing the global oxygen isotope proxy to ice volume and meridional position of the ITCZ to changes in trophic level of carbonate ecosystems. The results show that the development of Palaeogene Mediterranean photozoan assemblages coincides with periods when the ITCZ had shifted away from North Africa (as is the case presently), whereas the heterozoan assemblages thrived during increased nutrient flux when the precipitation belt was located over the Sahara. The climatic controls resulted in facies characteristics that exert a fundamental influence on porosity in carbonate reservoirs. Downloaded from fig. 2. Location and name of Oligocene formations referred to in text. Biotas include coralline red algae (CRA), large benthic foraminifera (LBF), bryozoans (Bry) and coral recorded in outcrops from the inner, mid-and outer platform environment.

New insights into the genesis of the Miocene collapse structures of the island of Gozo (Malta, central Mediterranean Sea)

“…The Late Triassic anhydrite beds recorded in Madonna Taz-Zejt 1ST1 borehole have a cumulative thickness of around 230 m, but occur below 4.5 km depth (Debono et al 2000). The relative shallow gypsum bed found 100-150 m below the base of the Lower Coralline Limestone in the Malta Platform is just 1 m thick (Gatt & Gluyas 2012). These data challenge the evaporite dissolution hypothesis.…”

“…Sea-level falls of c. 50-60 m have been reported in the global sea-level curves at the Eocene-Oligocene boundary and in the Middle Oligocene (Miller et al 2011). Relative sealevel falls of 10-20 m have been reported over the Malta Platform in the early Chattian period by Gatt & Gluyas (2012). However, none appear to have been sufficient to place western Gozo more than a few metres above sea level.…”

“…In Zabbar and Aqualta boreholes, drilled in the Malta Platform (Fig. 1a), a 1 m thick gypsum bed has been recorded at the Eocene-Oligocene boundary, 100-150 m below the base of the Lower Coralline Limestone (Gatt & Gluyas 2012).…”

“…However, the palaeohydrogeological model responsible for the karstification of evaporites is difficult to explain owing to the shortage of data. A model involving groundwater discharge into the sea cannot be invoked because the study area was an isolated carbonate platform when the Gozitan palaeosinkholes started to form (Gatt & Gluyas 2012). A possible conceptual model adapted to this situation is a subjacent dissolution related to focused vertical hydrothermal flow favoured by seismic activity (see Bertoni & Cartwright 2005, for a detailed explanation).…”

An investigation of porosity–velocity relationships in faulted carbonates using outcrop analogues

“…Miliolids are more typical of shallow-water, lower energy environments, but are easily transported into higher energy areas (Hallock & Glenn 1986;BouDagher-Fadel 2008). Gatt & Gluyas (2012) suggested the miliolid foraminifera in Malta represent a water depth of less than 10 m. As such, the Xlendi Member represents part of the inner ramp, both high-energy barrier shoals and the back-barrier environment.…”