Facies analysis and palaeoenvironmental interpretation of the Late Oligocene Attard Member (Lower Coralline Limestone Formation), Malta

Abstract

The Oligocene represents a key interval during which coralline algae became dominant on carbonate ramps and luxuriant coral reefs emerged on a global scale. So far, few studies have considered the impact that these early reefs had on ramp development. Consequently, this study aimed at presenting a highresolution analysis of the Attard Member of the Lower Coralline Limestone Formation (Late Oligocene, Malta) in order to decipher the internal and external factors controlling the architecture of a typical Late Oligocene platform. Excellent exposures of the Lower Coralline Limestone Formation occurring along continuous outcrops adjacent to the Victoria Lines Fault reveal in detail the three-dimensional distribution of the reef-associated facies. A total of four sedimentary facies have been recognized and are grouped into two depositional environments that correspond to the inner and middle carbonate ramp. The inner ramp was characterized by a very high-energy, shallow-water setting, influenced by tide and wave processes. This setting passed downslope into an inner-ramp depositional environment which was colonized by seagrass and interfingered with adjacent areas containing scattered corals. The middle ramp lithofacies were deposited in the oligophotic zone, the sediments being generated from combined in situ production and sediments swept from the shallower inner ramp by currents. Compositional characteristics and facies distributions of the Attard ramp are more similar to the Miocene ramps than to those of the Eocene. An important factor controlling this similarity may be the expansion of the seagrass colonization within the euphotic zone. This expansion may have commenced in the Late Oligocene and was associated with a concomitant reduction in the aerial extent of the larger benthonic foraminifera facies. Stackingpattern analysis shows that the depositional units (parasequences) at the study section are arranged into transgressive-regressive facies cycles. This cyclicity is superimposed on the overall regressive phase recorded by the Attard succession. Furthermore, a minor highstand (correlated with the Ru4/Ch1 sequence) and subsequent minor lowstand (Ch2 sequence) have been recognized. The biota assemblages of the Attard Member suggest that carbonate sedimentation took place in subtropical waters and oligotrophic to slightly mesotrophic conditions. The apparent low capacity of corals to form wave-resistant reef structures is considered to have been a significant factor affecting substrate stability at this time. The resulting lack of resistant mid-ramp reef frameworks left this zone exposed to wave and storm activity, thereby encouraging the widespread development of coralline algal associations dominated by rhodoliths.

Boulder accumulations related to extreme wave events on the eastern coast of Malta

“…As illustrated in Fig. 2, it includes five formations: (1) Lower Coralline Limestone formation (LCL), consisting of late Oligocene (Brandano et al, 2009) bioclastic limestones; (2) Globigerina Limestone formation (GLO), late Oligocene to middle Miocene in age (Baldassini et al (2013), Baldassini and Di Stefano (2015) and reference therein), consisting of pelagic marly limestones. It is subdivided, based on the occurrence of phosphoritic conglomerate beds (Baldassini and Di Stefano, 2015), into three members; (3) Blue Clay formation (BC), middle to late Miocene in age (Giannelli and Salvatorini, 1975;Hilgen et al, 2009), formed by silty marlstones; (4) Greensand formation, late Miocene (Tortonian) in age (Giannelli and Salvatorini, 1975), consisting of greenish marly/clayey glauconite sands and arenites; (5) Upper Coralline Limestone formation (UCL), late Miocene in age (Giannelli and Salvatorini, 1975), consisting of shallow-water bioclastic limestone deposits.…”

Biostratigraphy and paleoecology of the Oligo-Miocene Asmari Formation in the Izeh zone (Zagros Basin, SW Iran)

“…A similar assemblage was reported from the inner ramp of the Miocene sediments of the Central Apennines (Corda and Brandano, 2003) and from the Early Oligocene deposits of the Lower Inn Valley (Nebelsick et al, 2001). The abundance of larger benthic foraminifera (Borelis, Peneroplis, Dendritina, Heterostegina, Operculina) and zooxanthellate corals indicate mesophothic to euphotic condition (Halfar et al, 2004;Pomar et al, 2004;Pomar and Hallock, 2008;Brandano et al, 2009b;Mazzucchi and Tomassetti, 2011;Tomassetti and Brandano, 2013). During the Burdigalian a slightly restricted lagoon became established over the entire width of the platform in the SW (e.g.…”

Facies Interpretation, Depositional Environment and Sequence Stratigraphy of the Sartakht Formation in the Bakhshi Section, Located in Kalmard Block, East-Central Iran

“…Microfacies of this facies belt share such characteristics as presence of non-skeletal grains including pellet and lagoonal bioclasts such as benthic foraminifera, pelecypoda and green algae, capable of surviving in restricted and semi-restricted conditions [24]. The first common characteristics among microfacies of restricted lagoon environment is the presence of tranquility in their formation environment, and the second one is the high level of salinity in the depositional environment of the sediments of this group [23].…”