The interaction of tectonics, climate and eustasy in controlling dolomitization: A case study of Cenomanian–Turonian, shallow marine carbonates of the Iberian Basin

Abstract

During the Cretaceous, high global sea‐level and low latitudinal temperature variations led to the growth of epeiric carbonate platforms. Platform‐scale dolomitization of these platforms is not common, reflecting the low Mg/Ca ratio of seawater and a humid climate. This study describes the processes governing pervasive dolomitization of a land‐attached carbonate platform within the Iberian Basin. Dolomite is planar to sub‐planar with a geochemical signature consistent with dolomitization from penesaline seawater. Dolomitization was most pervasive during a 1 Myr period in the middle Cenomanian, by repeated reflux of seawater from brine pools formed on the top of a southward‐prograding carbonate platform. Tilting and structural reorganization in the Upper Cenomanian led to a reversal in polarity of the platform, and dolomitization was restarted by the northward reflux of seawater. Rising relative sea‐level and oceanic acidification led to back‐stepping of the platform such that the supply of dolomitizing fluids was cut off. In the Lower Turonian, pervasively dolomitized rudist rudstone facies in the south of the study area indicate that dolomitization restarted, either penecontemporaneously or later, from highly evaporated Campanian–Maastrichtian seawater. A systematic increase in dolomite crystal size up‐section ties broadly, but not entirely, to stratigraphy. It is possible that these textural differences reflect changes in fluid chemistry, limestone permeability or precursor rock texture. However, the lack of stratigraphic conformance, and the preservation of the earliest‐formed dolomite only in the oldest sediments, could indicate a progressive recrystallization of early‐formed dolomite through repeated reflux of brines. As such, the succession appears to preserve a fossilized record of dolomite recrystallization through time during the Cenomanian–Turonian. The results of this study therefore provide a record of the progressive dolomitization of a carbonate platform and demonstrate the important interplay of climate and basin‐scale tectonics on dolomite distribution and crystallinity.

Regional fault-controlled shallow dolomitization of the Middle Cambrian Cathedral Formation by hydrothermal fluids fluxed through a basal clastic aquifer

“…aquifer (e.g., Martín-Martín et al, 2015;Hollis et al, 2017;Lukoczki et al, 2019;Newport et al, 2020;Stacey et al, 2020). In totality, the results demonstrate the importance of a holistic, multi-scale evaluation of the post-depositional alteration of carbonate sedimentary rocks.…”

High‐magnesium calcite skeletons provide magnesium for burrow‐selective dolomitization in Cretaceous carbonates

“…In reported cases of burrow‐selective dolomitization during periods with low seawater Mg/Ca ratios, depositional environments are the restricted lagoon or tidal flat settings with high seawater salinity and sufficient Mg supply. Additionally, host rocks in these environments are primarily composed of lime mudstone or wackestone causing the permeability superiority of burrows over surrounding matrices, which contributes to the preferential fluid flux and dolomitization within burrows (Morrow, 1978; Saller & Yaremko, 1994; Rameil, 2008; Baniak et al ., 2014; Newport et al ., 2020; Rahim et al ., 2020; Alibrahim et al ., 2021; Ye et al ., 2021; Yang et al ., 2022a). By contrast, the Rumaila Formation was deposited in an open marine with normal seawater salinity and the host rocks were dominantly packstone.…”

“…Statistically significant correlations exist between seawater Mg/Ca ratios and dolomite abundance to some extent (Wilkinson & Algeo, 1989;Burns et al, 2000;Gabellone & Whitaker, 2016;Newport et al, 2020). Therefore, the low Mg/Ca ratio of seawater during the Cretaceous is believed to be linked to the scarcity of early dolomitization in coeval carbonate platforms (Newport et al, 2020).…”

“…Secular oscillations in seawater Mg/Ca ratios were observed during the Phanerozoic (Chilingar, 1956; Wilkinson & Algeo, 1989; Hardie, 1996; Lowenstein et al ., 2001; Horita et al ., 2002). Statistically significant correlations exist between seawater Mg/Ca ratios and dolomite abundance to some extent (Wilkinson & Algeo, 1989; Burns et al ., 2000; Gabellone & Whitaker, 2016; Newport et al ., 2020). Therefore, the low Mg/Ca ratio of seawater during the Cretaceous is believed to be linked to the scarcity of early dolomitization in coeval carbonate platforms (Newport et al ., 2020).…”

Coevolution of diagenetic fronts and fluid-fracture pathways

“…Although transitions might appear sharp in outcrop, detailed petrographic and geochemical analysis reveals that they are not simple, but the result of multiple, potentially related events. Poorly ordered, non-stoichiometric dolomite that forms in near surface settings is recrystallized 39 , either downwards in refluxing systems 57 or laterally, from faults. This can result in a back-stepping of the reaction front in which the reaction fronts retreating closer to the fluid source or fracture corridors as porosity is progressively occluded through subsequent dolomitization 5 .…”