A lost Tethyan evaporitic basin: Evidence from a Cretaceous hemipelagic meta‐selenite – red chert association in the Eastern Mediterranean realm

Abstract

Ancient evaporite deposits are geological archives of depositional environments characterized by a long-term negative precipitation balance and bear evidence for global ocean element mass balance calculations. Here, Cretaceous selenite pseudomorphs from western Anatolia ('Rosetta Marble')characterized by their exceptional morphological preservationand their 'marine' geochemical signatures are described and interpreted in a process-oriented context. These rocks recorded Late Cretaceous high-pressure/low-temperature, subductionrelated metamorphism with peak conditions of 1Á0 to 1Á2 GPa and 300 to 400°C. Metre-scale, rock-forming radiating rods, now present as fibrous calcite marble, clearly point to selenitic gypsum as the precursor mineral. Stratigraphic successions are recorded along a reconstructed proximal to distal transect. The cyclical alternation of selenite beds and radiolarian ribbon-bedded cherts in the distal portions are interpreted as a two type of seawater system. During arid intervals, shallow marine brines cascaded downward into basinal settings and induced precipitation. During more humid times, upwelling-induced radiolarian blooms caused the deposition of radiolarite facies. Interestingly, there is no comparable depositional setting known from the Cenozoic world. Meta-selenite geochemical data (d 13 C, d 18 O and 87 Sr/ 86 Sr) plot within the range of reconstructed middle Cretaceous seawater signatures. Possible sources for the 13 C-enriched (mean 2Á2&) values include methanogenesis, gas hydrates and cold seep fluid exhalation. Spatially resolved component-specific analysis of a rock slab displays isotopic variances between meta-selenite crystals (mean d 13 C 2Á2&) and host matrix (mean d 13 C 1Á3&). The Cretaceous evaporite-pseudomorphs of Anatolia represent a basin wide event coeval with the Aptian evaporites of the Proto-Atlantic and the pseudomorphs share many attributes, including lateral distribution of 600 km and stratigraphic thickness of 1Á5 to 2Á0 km, with the evaporites formed during the younger Messinian salinity crisis. The Rosetta Marble of Anatolia may represent the best-preserved selenite pseudomorphs worldwide and have a clear potential to act as a template for the study of meta-selenite in deep time. 2627

On the delimitation of the carbonate burial realm

“…Moreover, aragonite is the stable modification of CaCO 3 in the high-pressure metamorphic domain above ca 0.3 GPa (30 000 bar), a value significantly above the pressure regime typical of even the deep-burial domain. In regions where oceanic basins and their sedimentary inventory were largely destroyed in subduction zones, preserved and exhumed portions of high-pressure meta-carbonates may display evidence of retrograde conversion of aragonite to calcite and, under favourable conditions, yield highly relevant information for plate tectonic and palaeoenvironmental reconstructions (see discussion in Scheffler et al, 2015Scheffler et al, , 2019.…”

Early silicification of the Cyrenaican chert, Libya: The importance of moganite as a transitional silicon dioxide phase

“…These studies enforced the notion that the occurrence of these minerals could be taken as evidence of an evaporite precursor. Evaporites and length‐slow chalcedonic minerals are also found in deep marine pelagic sediments, and the latter in high Mg‐calcite shells found in turbidites (Jacka, 1974; Keene, 1983; Scheffler et al ., 2019), indicating that length‐slow silica minerals may also develop under non‐evaporitic conditions.…”