Abstract
Dolomitization is one of the most significant diagenetic reactions in carbonate systems, occurring where limestone (CaCO3) is replaced by dolomite (CaMg (CO3)2) under a wide range of crystallization temperatures and fluids. The processes governing its formation have been well studied, but the controls on the position of dolomitization fronts in ancient natural settings, particularly in a fault-controlled hydrothermal system (HTD), have received remarkably little attention. Hence, the origin and evolution of HTD dolomitization fronts in the stratigraphic record remain enigmatic. Here, a new set of mineralogical and geochemical data collected from different transects in a partially dolomitized Cambrian carbonate platform in western Canada are presented to address this issue. Systematic patterns of sudden decrease in the magnesium content (mol% MgCO3) and increase in porosity were observed towards the margin of the body. Furthermore, fluid temperatures are cooler and δ18Owater values are less positive at the dolomitization front than within the core of the body. These changes coincide with a change from poorly ordered, planar-e dolomite with multiple crystal zonations at the margin, to an unzoned, well-ordered, interlocking mosaic of planar-s to nonplanar dolomite in the core of the body.These phenomena are hypothesized to reflect dynamic, self-limiting processes in the formation and evolution of HTD dolomitization fronts through (i) plummet of dolomitization potential at the head of dolomitizing fluids due to progressive consumption of magnesium and fluid cooling; and (ii) retreat of dolomitization fronts towards the fluid source during subsequent recrystallization of the dolomite body, inboard of the termination, once overdolomitization took place. This new insight illustrates how dolomitization fronts can record the oldest phase of dolomitization, instead of the youngest as is often assumed. Formation of porosity is interpreted to occur as the result of acidification-induced grain leaching during the development of dolomitization fronts. This mechanism, coupled with retrogradation of dolomitization fronts, may help to explain the apparent enhancement of porosity in proximity to dolomitization fronts.
Highlights
The presence of dolomite bodies and their associated depositional and diagenetic contacts with the adjacent limestone have been widely reported in the literature across a range of dolomitized platforms, using field studies (e.g. Wilson et al, 1990; Nader et al, 2007; Budd and Mathias, 2015; Hollis et al, 2017; Al-Ramadan et al, 2019) and numerical simulations (e.g. Xiao and Jones, 2007; Yapparova et al, 2017)
While dolomitization fronts occur in all dolomitized carbonates, they are most described in fault-controlled hydrothermal (HTD) systems because they are defined by an obvious color contrast with the adjacent limestone (e.g. Sharp et al, 2010; Merino and Canals, 2011)
This study aims to address this knowledge gap through systematic high-resolution sampling and analysis of several transects of Cambrian HTD dolomitization fronts in the Western Canada Sedimentary Basin (WCSB) (Fig. 1)
Summary
The presence of dolomite bodies and their associated depositional and diagenetic contacts with the adjacent limestone (i.e. dolomitization fronts) have been widely reported in the literature across a range of dolomitized platforms, using field studies (e.g. Wilson et al, 1990; Nader et al, 2007; Budd and Mathias, 2015; Hollis et al, 2017; Al-Ramadan et al, 2019) and numerical simulations (e.g. Xiao and Jones, 2007; Yapparova et al, 2017). Wilson et al, 1990; Nader et al, 2007; Budd and Mathias, 2015; Hollis et al, 2017; Al-Ramadan et al, 2019) and numerical simulations Wilson et al (1990) ascribed the position of dolomitization fronts to magnesium depletion due to cooling of the fluid. Numerical simulation by Merino and Canals (2011) further examined the formation of high temperature dolomitization fronts and proposed that the governing process was self-accelerating and induced by the release of Ca2+ during replacement. Budd and Mathias (2015) interpreted self-organization during dolomitization as a possible control on dolomite body termination in Miocene carbonates, based on the discrepancy in statistical trends between dolomite and limestone. Kondratiuk et al (2015) proposed that the formation of porosity at dolomitization fronts occurred through synchronization between dissolution and precipitation fronts
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