Selective Dolomitization By Syntaxial Overgrowth Around Detrital Dolomite Nuclei: A Case From the Jurassic of the Ligurian Brianconnais (Ligurian Alps)

Abstract

Dolomite crystals, occurring in Middle Jurassic dolostones and dolomitic limestones of theLigurian Briançonnais Domain (French-Italian Ligurian Alps), show irregular, complex zoning evidenced by cathodoluminescence (CL) and backscattered electron imaging. Comparable zoning patterns from other study areas have been attributed alternatively to recrystallization or dissolutionprecipitation processes. However, none of these mechanisms can account for the range of observations from the Ligurian Briançonnais Domain, which include: 1) the presence of multiple core types, each with different CL characteristics; 2) marked compositional differences between cores and surrounding rims (ferroan vs. nonferroan, Mg/Ca ratio); 3) a marked shape difference between cores, with irregular outlines, and rims which progressively approach a well-developed rhombohedral habit. These features are most reasonably interpreted as dolomitic rims that formed as syntaxial overgrowths around detrital dolomite nuclei, which in turn originated from erosion of underlying Triassic rocks and triggered selective replacement of fine-grained calcareous sediment.Failure to recognize the exact nature of this type of dolomite crystal may lead to overestimating the degree of dolomitization and to overlooking possible detrital inputs in a basin otherwise supplied with pure allochemical sediments, with consequent loss of information on the tectonosedimentary evolution.

Geological setting of the southern termination of Western Alps

“…The latter was affected only by minor low-angle faults along the weakest stratigraphic layers. This allowed the preservation of most of the primary features (Piana et al 2009;Bertok et al 2011Bertok et al , 2012Martire et al 2014) such as Cretaceous palaeoescarpments and palaeofaults that controlled the partitioning of the Alpine deformation, leading to the individuation of different-order sub-units.…”

Eolian dust influx and massive whitings during the kozlowski/Lau Event: carbonate hypersaturation as a possible driver of the mid-Ludfordian Carbon Isotope Excursion

“…-The common sorting between dolomite and quartz, along with the distinct signs of abrasion and incorporation of dolomite grains into composite grains, indicate a detrital origin of the dolomite. Some of the grains (with syntaxial overgrowths) contain a surface layer of the diagenetic, secondary ferroan dolomite formed around the Fe-poor detrital nuclei (see Martire et al 2014).…”

“…Loope & Haverland 1988). Despite these similarities, the appearances of detrital dolomite are not limited to carbonate eolianites (see Martire et al 2014 and references therein). Hence, the mineralogical composition and sorting cannot be conclusive alone for the eolian transport of the detrital dolomite, which could also be derived by fluvial input (e.g.…”

Geology of the Entracque–Colle di Tenda area (Maritime Alps, NW Italy)

“…This document is, thus, the first published detailed map as part of a scientific programme focusing on the revision of the stratigraphic and tectonic setting of a large area encompassing, from NW to SE, the Italian side of the Maritime Alps and the westernmost part of the Ligurian Alps, the main results of which have been recently published (Barale et al, 2013;Bertok, Martire, Perotti, d'Atri, & Piana, 2011d'Atri, Piana, Barale, Bertok, & Martire, submitted;Martire, Bertok, d'Atri, Perotti, & Piana, 2014;Perotti et al, 2012;Piana et al, 2009Piana et al, , 2014.…”