Diagenesis of Oligocene continental sandstones in salt-walled mini-basins-Sivas Basin, Turkey
Résumé
The recent discovery of Oligo-Miocene salt-walled continental mini-basins in the Sivas Basin (central Anatolia, Turkey) provides the opportunity to unravel the influence of halokinesis on the diagenesis of continental mini-basin infilling. In this study, petrographic and geochemical analyses are used to define the diagenetic sequences recorded by two mini-basins filled mainly by fluvial clastic sediments of the upper Oligocene Karayün Formation. The initial diagenetic features are those commonly encountered in arid to semi-arid continental environments, i.e. clay infiltration, hematite precipitation and vadose calcite cement. Other early cements were strongly controlled by sandstone detrital composition in the presence of saline/alkaline pore water. In feldspathic litharenites and lithic arkoses, near-surface alterations were characterized by the precipitation of analcime (up to 10%), albite and quartz overgrowths (<. 1%). These events were followed by extensive calcite cementation (up to 30%) during shallow burial diagenesis which prevented further mesogenetic alteration phenomena such as compaction. In feldsarenites, early diagenesis differs by (i) the absence of analcime, (ii) better developed albite cements, (iii) thin smectite-illite coatings forming pore linings and (iv) patchy calcite cementation (<. 5%). The limited development of calcite cement allowed mesogenetic alterations to occur, such as late quartz overgrowths, albitization of feldspar grains and chemical compaction. All these phases are responsible for the low porosity of feldsarenites (<. 2%). The greater abundance of carbonate cement in feldspathic litharenites and lithic arkoses is related to a greater proportion of detrital limestone in these sandstones. Early precipitation of analcime, albite, smectite-illite and quartz was likely triggered by the alteration of reactive grains by near-surface saline/alkaline brines originating from the dissolution of adjacent diapiric structures. Mini-basin confinement resulting from halokinesis was probably an important factor influencing surface and subsurface saline/alkaline fluid flow and related diagenesis. Despite the evident role of detrital composition in controlling the recorded diagenetic evolution pathways, the striking similarity between observations in the Sivas Basin and equivalent halokinetic settings of the Pre-Caspian domain suggests that continental clastic deposits of mini-basins may exhibit common diagenetic alteration effects, especially near-surface zeolite precipitation. These effects are linked to the close proximity of diapiric salt during deposition and burial, and result in a significant degradation of porosity. © 2016 Elsevier B.V.