Mikrofacje i diageneza wapieni oolitowych najniższego triasu w profilu Gorzów Wielkopolski IG 1 (zachodnia Polska)
Abstract
Microfacies and diagenesis of the lowermost Triassic oolitic limestones from the Gorzów Wielkopolski IG1 borehole (western Poland).
A b s t r a c t. Lowermost Triassic oolitic limestones occur in the Gorzów Wielkopolski IG 1 borehole as thin intercalations (mostly less than 1 m thick)within laminated or massive reddish brown mudstones or sandy mudstones of the Baltic and Pomerania formations of the Lower and Middle Buntsandstein, respectively. In order to characterize the microfacies and diagenesis of the limestones, 46 thin sections from 44 core specimens were investigated. The limestone intercalations are poorly diversified in terms of microfacies, especially in the lower part of the section (Baltic Formation), where almost exclusively fine-grained, well-sorted ooid grainstones occur. The upper part of the section (Pomerania Formation) is more diverse. In addition to grainstones similar to those of the Baltic Formation, coarse-grained and/or poorly sorted ooid grainstones, subordinate stromatolites, oncoid grainstones, and mudstones are present. The limestones are almost completely devoid of bioclasts. The only organic remains present in the thin sections are single rounded plates composed of apatite (possibly fish scales) and, even rarer, snail shells and small shells of other organisms (bivalves?, ostracods?), now dissolved (probably originally aragonitic) and visible only due to the preserved coatings of micritic calcite cement previously precipitated on them. The diagenesis of these deposits has not been very intensive and involves mainly cementation by calcite and subordinately by anhydrite, recrystallization of some ooids and other grains (including carbonate bioclasts), and to a lesser extent by dissolution under pressure.
A b s t r a c t. Lowermost Triassic oolitic limestones occur in the Gorzów Wielkopolski IG 1 borehole as thin intercalations (mostly less than 1 m thick)within laminated or massive reddish brown mudstones or sandy mudstones of the Baltic and Pomerania formations of the Lower and Middle Buntsandstein, respectively. In order to characterize the microfacies and diagenesis of the limestones, 46 thin sections from 44 core specimens were investigated. The limestone intercalations are poorly diversified in terms of microfacies, especially in the lower part of the section (Baltic Formation), where almost exclusively fine-grained, well-sorted ooid grainstones occur. The upper part of the section (Pomerania Formation) is more diverse. In addition to grainstones similar to those of the Baltic Formation, coarse-grained and/or poorly sorted ooid grainstones, subordinate stromatolites, oncoid grainstones, and mudstones are present. The limestones are almost completely devoid of bioclasts. The only organic remains present in the thin sections are single rounded plates composed of apatite (possibly fish scales) and, even rarer, snail shells and small shells of other organisms (bivalves?, ostracods?), now dissolved (probably originally aragonitic) and visible only due to the preserved coatings of micritic calcite cement previously precipitated on them. The diagenesis of these deposits has not been very intensive and involves mainly cementation by calcite and subordinately by anhydrite, recrystallization of some ooids and other grains (including carbonate bioclasts), and to a lesser extent by dissolution under pressure.