Examining the interplay of climate and low amplitude sea‐level change on the distribution and volume of massive dolomitization: Zebbag Formation, Cretaceous, Southern Tunisia

Abstract

During the Cretaceous, a humid global climate, calcitic seas, high relative sealevel and low amplitude changes in relative sea-level largely prevented largescale dolomitization in many carbonate successions. However, the well-exposed shallow-water carbonate sediments of the Upper Albian-Lower Turonian Zebbag Formation on the Jeffara Escarpment, southern Tunisia, are pervasively dolomitized. This study considers why dolomitization was so widespread in this region during a period of Earth history when platform-scale dolomitization is rare. Marine conditions were established in the Upper Albian, evidenced by stacked upward-shallowing packages of shallow subtidal to peritidal carbonate sediments in the basal Rhadouane Member. A gradual increase in the volume of subtidal sediments in the Cenomanian Kerker Member, culminated in deposition of laterally extensive marls, during maximum flooding of the platform in the Lower Turonian. The overlying Gattar Member was then deposited in shallower water as relative sea-level fell. The entire Zebbag Formation is pervasively replaced by stratabound, fabric-retentive, dolomite, except within the marl at the top of the Kerker Member, which is only partially dolomitized. Petrographic textures indicate dolomitization largely post-dated marine cementation and platform emergence but pre-dated chemical compaction. Slightly more positive oxygen isotope signatures, slightly elevated concentrations of Sr and a near-absence of evaporites are consistent with dolomitization by reflux of mesohaline sea water. An upward-decrease in major element concentrations and higher 87 Sr/ 86 Sr compared to Upper Cretaceous sea water suggest that basal, Albian siliciclastic beds acted as aquifers facilitating dolomitization by fluxing fluids offshore. Dolomitization is interpreted to have resulted from multiple fluxes of sea water over periods of 0Á5 to 2Á5 Ma. The unusually high volume of dolostone for a platform of this age most probably reflects deposition within an arid climate belt, where an efficient reflux system was facilitated by basal, permeable siliciclastic strata.

Early and pervasive dolomitization by near‐normal marine fluids: New lessons from an Eocene evaporative setting in Qatar

“…This is consistent with other studies that have analyzed evaporite-associated dolomites and provided data that suggest near-normal seawater was responsible for dolomitization (e.g. Newport et al, 2017;Manche & Kaczmarek, 2019). For example, Newport et al (2017) analyzed the Upper Albian-Lower Turonian Zebbag Formation in southern Tunisia.…”

“…The Zebbag Formation consists of metre-scale shallowing-upward peritidal and subtidal cycles, with a 1 m thick gypsum horizon in the middle Kerker Member. Although not present in the study by Newport et al (2017), the Zebbag is also overlain by bedded evaporites north of the authors' study area (Newport et al, 2017). Based on facies distributions, dolomite texture variations, slightly positive isotope signatures, slightly elevated Sr concentrations and a nearabsence of evaporites, Newport et al (2017) hypothesized that the Zebbag Formation was pervasively dolomitized by reflux of mesohaline seawater.…”

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

“…During migration, fluid interaction with clays within the sandstone would have enriched concentrations of radiogenic strontium and Fe within the brine, and this is reflected in the 87 Sr/ 86 Sr and Fe concentrations of the dolostone. A similar relationship between a basal clastic aquifer and pervasive dolomitization of a carbonate platform was also noted by Newport et al (2017), suggesting that the presence of an underlying sandstone facilitated the flux of dolomitizing brines.…”

“…Dolomitization by penesaline brines (72 to 199&) has been interpreted in several studies (e.g. Qing et al, 2001;Benito & Mas, 2007;Iannace et al, 2011Iannace et al, , 2013Rott & Qing, 2013;Newport et al, 2017), and therefore it is possible that seawater was sufficiently saline to sink and reflux through the sediment pile. Reactive transport models (RTM) demonstrate that it is possible to dolomitize large volumes of limestone by penesaline brines (Jones & Xiao, 2005;Al-Helal et al, 2012;Gabellone & Whitaker, 2016), although multiple phases of fluid flux are required to cause pervasive dolomitization of the sediment stack (Garcia-Fresca et al, 2009.…”

Petroleum reservoir quality prediction: overview and contrasting approaches from sandstone and carbonate communities

“…The brine sinks and refluxes seaward through the sediment pile, with dolomitization being facilitated by the slight increase in temperature and occurring preferentially in sediments with a high reactive surface area and/or a 'seed' (nucleation point) for dolomitization, such as high-Mg calcite (Jones & Xiao 2005;Jiang et al 2014). It is not always necessary for brines to have reached gypsum saturation for reflux dolomitization to occur since there simply needs to be repeated flux of seawater (Garcia-Fresca et al 2012;Newport et al 2017). Typical features of platform-scale dolomitization by reflux include stratabound, fabric-selective and sometimes fabric retentive dolomitization, all at temperatures of less than c. 608C.…”