Early diagenetic dolomitization and dedolomitization of Late Jurassic and earliest Cretaceous platform carbonates: A case study from the Jura Mountains (NW Switzerland, E France)

Abstract

Early diagenetic dolomitization is a common feature in cyclic shallow-water carbonates throughout the geologic record. After their generation, dolomites may be subject to dedolomitization (re-calcification of dolomites), e.g. by contact with meteoric water during emersion. These patterns of dolomitization and subsequent dedolomitization frequently play a key role in unravelling the development and history of a carbonate platform. On the basis of excellent outcrops, detailed logging and sampling and integrating sedimentological work, high-resolution sequence stratigraphic interpretations, and isotope analyses (O, C), conceptual models on early diagenetic dolomitization and dedolomitization and their underlying mechanisms were developed for the Upper Jurassic / Lower Cretaceous Jura platform in north-western Switzerland and eastern France. Three different types of early diagenetic dolomites and two types of dedolomites were observed. Each is defined by a distinct petrographic/isotopic signature and a distinct spatial distribution pattern. Different types of dolomites are interpreted to have been formed by different mechanisms, such as shallow seepage reflux, evaporation on tidal flats, and microbially mediated selective dolomitization of burrows. Depending on the type of dolomite, sea water with normal marine to slightly enhanced salinities is proposed as dolomitizing fluid. Based on the data obtained, the main volume of dolomite was precipitated by a reflux mechanism that was switched on and off by high-frequency sea-level changes. It appears, however, that more than one dolomitization mechanism was active (pene)contemporaneously or several processes alternated in time. During early diagenesis, percolating meteoric waters obviously played an important role in the dedolomitization of carbonate rocks that underlie exposure surfaces. Cyclostratigraphic interpretation of the sedimentary succession allows for estimates on the timing of early diagenetic (de)dolomitization. These results are an important step towards a better understanding of the link between high-frequency, probably orbitally forced, sea-level oscillations and early dolomitization under Mesozoic greenhouse conditions.

Dolomitization of the lower St. George Group on the Northern Peninsula in western Newfoundland: implications for lateral distribution of porosity

“…3d), suggesting a change in the redox conditions and chemistry of the parent dolomitizing fluid during the course of crystallization (e.g. Rameil, 2008;Kırmacı, 2008). The same features have been also documented for the D2 phase of the Isthmus Bay section (Azmy et al, 2009;Conliffe et al, 2009), although the D2 crystals of the Isthmus Bay section were relatively smaller compared to those in the investigated core and do not exhibit sweeping extinction.…”

“…The crystals vary in size between about 50 and 250 µm and exhibit cloudy cores with clear rims (e.g. Azmy et al, 2008Azmy et al, , 2009Kırmacı, 2008;Rameil, 2008;Conliffe et al, 2009) under plane polarized light ( Fig. 3b-d) and concentric zoned luminescence under CL (Fig.…”

Relationships between sequence stratigraphy and diagenesis of corals and foraminifers in the Middle Eocene, northern Egypt

“…In the studied sections, the transformation of pore water chemistry from Fretwell et al, 2005). A similar interpretation for the dedolomitization process was suggested by Rameil (2008). He concluded that this type of dedolomitization occurs after long-term emersion events (e.g., large-scale/"3rd-order" SBs).…”

Origin of Lower Ordovician dolomites in eastern Laurentia: Controls on porosity and implications from geochemistry

“…The cloudy core with clear rims of D2 crystals indicate replacement of a precursor carbonate (cloudy core) and cementation (clear rim) whereas zoned CL images reflect chemical variations within the crystal (Warren, 2000). Undulose extinction in dolomites is caused by distorted crystal lattice that results from crystal growth at high temperatures (e.g., Warren, 2000;Rameil, 2008;Kırmacı, 2008). This is consistent with the mean Th (126.6 ± 12.8 o C, n ¼ 97) value of D2 and suggests crystallization at higher temperatures and deeper burial conditions relative to D1.…”