Genesis of Late Miocene-Pliocene lacustrine palygorskite and calcretes from Kırşehir, central Anatolia, Turkey

Abstract

AB ST R ACT : This paper addresses the mineralogy, geochemistry, and genesis of palygorskite and associated calcretes in the Kırs°ehir region. In this area, the Quaternary calcretes are present within and/or on lacustrine mudstones of the Kızılırmak Formation (Late Miocene-Pliocene). The calcretes occur in the form of nodules, tubes, fracture-infills, and hard laminated crusts (hardpans). Calcrete samples are predominantly composed of calcite associated with smectite, quartz, feldspar, and minor palygorskite. The lacustrine mudstone samples consist mainly of smectite and palygorskite with minor calcite, quartz, feldspar, illite, kaolinite and amorphous material. Petrographic studies revealed that the calcrete samples are predominantly micrites that were partially converted to microsparites by recrystallization. Scanning and transmission electron microscopy images indicate that palygorskite fibres formed authigenically on and between calcite rhombs in the calcrete samples and at the edges of smectite flakes of host rocks adjacent to the calcretes. The oxygen and hydrogen stable isotope values of the calcite suggest that it precipitated from meteoric water under arid or seasonally arid climatic conditions. Calcite precipitation occurred in the vadose zone, as evidenced by desiccation cracks and vadose pisolites. The enrichment of Rb+BaÔSr and Light Rare Earth Elements (LREE) relative to Heavy Rare Earth Elements (HREE)with a distinct negative Eu anomaly are responses to the fractionation of volcanogenic feldspar and hornblende under the influence of meteoric water. The palygorskite associated with calcretes was precipitated from alkaline water rich in Si, Al and Fe and low in Mg. The palygorskite precipitation occurred at increased temperatures, characterising the advanced stage of calcrete formation which lowers the Ca content in the soil water. The required chemical elements for palygorskite and calcite were most likely derived from the dissolution of smectitic clays, ignimbrites and carbonate minerals.The occurrence of palygorskite in calcretes has been studied extensively over the past 30 years (e.g.

Spanish palygorskites: geological setting, mineralogical, textural and crystal-chemical characterization

“…Usually, it is associated with carbonates, evaporites and chert and with other Mg-rich clays, such as saponite, kerolite and stevensite. Palygorskite is also described in calcretes as minor diagenetic constituent (Eren et al, 2008;Kaplan et al, 2013;Kadir et al, 2014). The best condition for its formation appears to be in lacustrine-palustrine environments, as repeatedly reported (Millot, 1970;Trauth, 1977;Singer, 1979;Galán & Castillo, 1984;Jones, 1986;Jones & Galán, 1988;Webster & Jones, 1994;Calvo et al, 1999;Akbulut & Kadir, 2003).…”

Hydrothermal Carbonate Mineralization, Calcretization, and Microbial Diagenesis Associated with Multiple Sedimentary Phases in the Upper Cretaceous Bekhme Formation, Kurdistan Region-Iraq

“…Well-developed features characterized the mature profile and were distinguished by thick "massive" indurated carbonate and clay accumulation horizons [91][92][93] with hard laminated crusts [66,89]. The immature profile was distinguished by less-developed horizons [66] and the lack of hard laminated crusts [22,94,95].…”

Fibrous clay minerals as lithostratigraphic markers in a Cenozoic continental basin (Sado Basin, Portugal)

“…The genesis of the palygorskite in sediments and soils has been well documented (Akbulut and Kadir, 2003;Galán and Castillo, 1984;Jamoussi et al, 2003;Kadir et al, 2014;Karakaş and Kadir, 1998;Rodas et al, 1994). Smectite is largely indicated as the palygorskite precursor Singer, 2002;Yeniyol, 2012).…”