Sedimentary facies analysis of a high-frequency, small-scale, peritidal carbonate sequence in the Lower Jurassic of the Tripolis carbonate unit (central western Crete, Greece): Long-lasting emergence and fossil laminar dolocretes horizons

Abstract

The study examines a Lower Jurassic (late Liassic) cyclic lagoonal–peritidal stratigraphic unit outcropping in central western Crete (Tripolis unit), which corresponds to the eastern (internal) part of the mainland Gavrovo-Tripolis platform, the most significant external platform of the Hellenides. The studied Tripolis carbonate sequence consists of meter-scale, shallowing-upward successions of restricted inner-carbonate platform facies, including cyclically repeated subtidal, intertidal and supratidal facies, that are separated by erosion surfaces (elementary cycles). Each cycle starts with relatively open-marine facies, which are overlain by shallower, more restricted facies (tidal flat progradation). The lithofacies association includes dolomitic intraclastic–peloidal–bioclastic wackestones–packstones/floatstones and grainstones/rudstones dominated by a restricted shallow-marine fauna (bivalves, gastropods, ostracods and seldom benthic foraminifers), representing a shallow subtidal to intertidal, moderately high-energy environment within an inner-platform setting (peritidal environment to restricted lagoon). This lithofacies association has been intermittently subaerially exposed and has undergone diagenetic processes in an inter- or supratidal environment, exhibiting features of vadose diagenesis and pedogenesis due to long-lasting exposure along certain horizons. The peritidal facies are capped by dolocretes controlled by root-activities (laminar dolocretes, peloidal–pisoid dolocretes and massive dolocretes), marking the end of each depositional cycle, and, thus, distinguishing the successive episodes of a prolonged subaerial exposure period and birth of paleosol horizons. Dolocretes consist a diagenetic facies, characterized by several vadose and pedogenic fabrics, including fenestral cavities with geopetal structures, “flower spar” to blocky sparry cement in primary pores, micritic coatings, crudely pelleted walls, alveolar-septal texture, infills of rootlet moulds, inter-granular micritic bridges, meniscus cement, sinuous desiccation cracks, circum-granular cracking and in-situ brecciation. The stratigraphic distribution of the cyclothems, suggests relative sea-level control on the peritidal cyclicity controlled by uniform tectonic subsidence and eustasy (allocyclic processes), in an inner- ramp-to-shelf carbonate setting with tidal flat and restricted lagoon depositional environment. However, a combination of allocyclic with autocyclic processes controlling accommodation space and sediment accumulation, should be also taken in consideration. The studied high-frequency cycles are interpreted to have been formed during the falling stage and correspond to lowstand systems tract (LST) sediments, which consists part of a regressive peak of the Lower/Middle Jurassic Transgressive–Regressive Facies Low Frequency Cycle (2nd order). Due to the absence of massive evaporites, dolomitization has been attributed to reflux of slightly increased-salinity marine fluids (i.e. penesaline), that are driven into the underlying lagoonal–peritidal carbonate sediments during periods of long term sea-level fall in a marginal marine setting.