Deposition and diagenesis of carbonate conglomerates in the Kremenara anticline, Albania: a paragenetic time marker in the Albanian foreland fold-and-thrust belt

摘要

This paper addresses the diagenesis of carbonate conglomerates in that it assesses the potential of conglomerates in refining the paragenetic history in complex structural areas, such as the Albanian foreland fold-and-thrust belt. Of major interest are stylolites (burial and tectonic) which are restricted to conglomerate fragments or which crosscut the conglomerate matrix. Based on the inferred age of stylolite development in relation to burial, uplift and tectonic history, and the Lower to Middle Miocene age of the conglomerates, the succession of diagenetic events was subdivided into several stages. The Pocem polymict transgressive carbonate conglomerate (Kremenara anticline, central Albania) was deposited in a shallow marine environment. These conglomerates are covered by intertidal rhodolithic packstones-grainstones. The stable-isotope signature of these packstones-grainstones (delta O-18(V-PDB) = -1.0 to +0.7 parts per thousand; delta C-13 = +1.0 to +1.4 parts per thousand) plots is within the range of marine Early and Middle Miocene values. Shortly after deposition of the conglomerates, micritization, geopetal infill and acicular calcite cementation took place. A first calcite vein generation is interpreted as having formed from a Messinian brine during shallow burial. Burial stylolites developed during further burial in the Pliocene. These stylolites serve as an important diagenetic time marker. The post-burial stylolite meteoric calcite vein cement probably precipitated during the following telogenetic stage. Karstification and calcite concretion precipitiation pre-date overturning of the western limb of the anticline. Reopening of subvertical fractures and tectonic stylolites in the western limb of the Kremenara anticline, followed by oil migration, represents one of the latest diagenetic events. These fractures and stylolites provide major pathways for hydrocarbon production.