A diagenetic study of the lower coralline limestone (oligocene), the Maltese Islands

[摘要] The Maltese Islands are located on a broad platform extending from northern Africa to Sicily which divides the Mediterranean Sea into an eastern and western basin. Strata exposed on the islands range from Upper Oligocene to Upper Miocene in age and are predominantly carbonates with one pelagic shale unit. This study focuses on the lowest formation, the Lower Coralline Limestone which is of Upper Oligocene (Chattian) age. The mid-Tertiary was a tectonically active time in the central Mediterranean. The western Mediterranean basin was forming during the Neogene and tectonic thrusting occurred to the west, north and east of the study area. Malta;;s location on a shallow, relatively stable platform in the center of a tectonically active Mediterranean places it in an interesting setting for diagenetic study. Unlike Recent carbonate sediments, the original mineralogic composition of the Lower Coralline Limestone was dominated by high magnesian and low magnesian calcite with only minor amounts of aragonite. Such a mineralogic assemblage would stabilize to low magnesian calcite rapidly and could conceivably affect early marine cementation and later episodes of fresh-water diagenesis. Fine-grained, fibrous marine cementation is present, but poorly preserved and limited to packstones and grainstones. Fresh-water, phreatic cements occurring in the Lower Coralline are more varied in crystal habit and abundant than the early marine cements. Three stages of meteoric cementation are recognized. An early period of syntaxial rim cements on echinoid fragments, accompanied or followed by grain compaction, forms the dominant cement type found in these rocks. These overgrowths display stratigraphically continuous luminescent zones like those reported in Mississippian limestones in New Mexico (Meyers, 1974) . The second phase of meteoric cementation produced fine- to medium-grained scal enohedra cement that clearly follows compaction and echinoid overgrowth development. These cements do not luminesce and constitute less than 20 percent of the fresh-water cements. The final stage of fresh-water cementation produced fine to medium grained, equant granular. void-filling spar. Diagenetic features examined in the Lower Coralline Limestone indicate at least two separate episodes of emergence and fresh-water cementation. Timing of emergent periods can be limited by extent of phreatic diagenesis within the lower units exposed on Malta. Stratigraphic relations of overlying formations that have been down-faulted into subsidence structures created by collapse of solution caverns within the Lower Coralline Limestone (Pedley, 1974) and limited erosional contacts in the lower part of the section provide possible times of fresh-water influence. In addition, proposed periods of subaerial exposure coincide closely with eustatic sea level drops described by Vail et al. (1978) .

[发布机构] Rice University