Origin, Mechanism and Distribution of Multi-Phase Dolomitization in the Upper Jurassic Arab Formation

Abstract

Dolomitization of ancient carbonate platforms is poorly understood partly because of the lack of modern analogues and the failure to synthesize ordered dolomite at low-temperatures. Challenges also remain uncertain with ongoing debate about whether single or multiple mechanisms drive extensive dolomite formation. Debate continues over whether the mineralogical, petrographic, and geochemical records of dolomite retain the pristine signature or have been overprinted by diagenetic processes. This dolomitization research of the Upper Jurassic Arab Formation integrates Mg isotope analysis of dolostones and in-situ U-Pb dating of dolomite, with C-O-Sr isotopes and element concentrations, as well as textural and mineralogical parameters, to investigate dolomitization conditions and evolution of ancient dolomite records during burial diagenesis.

Dolostones in the Upper Jurassic Arab Formation on the Arabian Plate were probably formed by sabkha-seepage reflux dolomitization, as supported by several lines of evidence: (1) Mg isotope values of the dolomitization fluids (-1.00 ‰, -0.65 ‰, -0.45 ‰, and -0.25‰) are similar to Late Jurassic Kimmeridgian-Tithonian seawater signatures (c. -0.55‰) and align with published values for evaporative sabkha dolostones, indicating Mg cations for Arab dolomite formation were delivered from ambient seawater; (2) more depleted Mg isotope signatures in dolomite formed at shallower depth than that formed at deeper depth, suggests a top-down dolomitization process which is controlled by percolation of dense and saline fluids, consistent with the advective flow model and related seepage reflux dolomitization; (3) the similarity between the δ13CDol-VPDB values (0.57 to 3.84 ‰; av. 2.67 ‰) and most 87Sr/86Sr ratios (0.706964 to 0.707691; av. 0.707212) of dolostones with ambient Kimmeridgian-Tithonian seawater isotope signatures; (4) in-situ U-Pb dating results of crystalline dolostones (151.4 ± 1.0 Ma / 1.6 Ma ), dolopackstone-dolograinstones (155.0 ± 1.2 Ma / 4.0 Ma and 145.61 ± 0.68 Ma / 0.80 Ma), and dolomudstone-dolowackestones (143.99 ± 0.82 / 1.28 Ma and 134.1 ± 1.5 Ma / 2.6 Ma ) align with Kimmeridgian-Tithonian depositional ages (154.8 ± 0.8 Ma to c. 145 Ma); and (5) abundant anhydrite cementation supports the impact of hot arid paleoclimate on dolomitization during the Late Jurassic.

Dolomitization in the Upper Jurassic Arab Formation reservoir can be constrained within 2nd, 3rd, and 4th order sequences. The formation represents a 2nd order regressive sequence/cycle (highstand systems tracts, HST) that was deposited across a carbonate ramp with shoal under hot arid climatic conditions during the Kimmeridgian-Tithonian. Deposition occurred subsequent to maximum marine transgression (maximum flooding surface, MFS) and was accompanied by a progressive increase in the degree of restriction of the connectivity between the inner platform (supratidal, upper intertidal and lagoon) and the open sea. Sporadic dolomitization took place in transgressive systems tracts (TST) of the 3rd and 4th order cycles of earliest stages of the 2nd order regression, below the parasequence boundaries and within bioturbation sites. Subsequent rapid and extended 2nd order marine regression cycles were embossed by trends of progressive increase in salinity, which resulted in concomitant systematic decrease in limestone deposition and increase in intensity of seepage reflux/sabkha dolomitization by seepage reflux of penesaline (salinity saturated with Ca-sulfate) /mesohaline (below Ca-sulfate saturation) brines. The reflux of the latter brines is attributed to smaller extent of relative sea-level falls, i.e., partial restriction of the inner platform. Dolomitization is most prevalent in the 4th order HST sequences and was accompanied by occlusion of the moldic and intercrystalline pores in the dolostones by gypsum/anhydrite cement. Dolomitization that was accompanied by cementation of gypsum/anhydrite in the late 2nd order HST is attributed to reflux of penesaline brines. Higher frequency changes in the relative sea level and selective dolomitization along bioturbation sites influenced the distribution patterns of dolostones.

The overprinting of dolomite initial petrographic, mineralogical and geochemical records during burial diagenesis varies between the upper and lower intervals of the Arab Formation. The limited alteration, such as dolomite recrystallization, in the upper interval is supported by: (1) the retention of precursor limestone textures in dolomudstones-wackestones and dolopackstones-grainstones; (2) more scattered dolomite stoichiometry values (45.23 mol % MgCO3 to 53.47 mol % MgCO3; av. 47.91 mol % MgCO3) and lower values in cation ordering (0.32 to 0.78; av. 0.57), with finer dolomite crystals; (3) relatively heavier but still depleted δ18ODol-VPDB values (-0.8 to -4.42 ‰; av. -1.52 ‰) compared to those in the lower interval (3.75 to -5.95 ‰; av. -4.85 ‰); (4) in-situ U-Pb dating ages of dolomudstones and dolograinstones consistent with the depositional ages indicating an early closure of U-Pb system in dolomite; and (5) extensive, laterally continuous anhydrite layers as compartmentalization for fluid circulation. On the contrary, the lower interval exhibits evidence of more substantial alteration by late diagenesis, such as recrystallization and burial cementation by basinal fluids, including: (1) obliterated depositional textures of precursor limestones and the coarser dolomite crystals, with mottled cathodoluminescence texture; (2) narrower variations of stoichiometry (44.33 mol % MgCO3 to 50.60 mol % MgCO3; av. 47.42 mol % MgCO3) and increased cation ordering values (0.39 to 1.92; av. 0.81); (3) considerably depleted δ18ODol-VPDB values (-3.75 to -5.95 ‰; av. -4.85 ‰), suggesting significant alteration during burial by increase in temperatures; (4) the relative enrichment of 87Sr/ 86Sr values in dolostones (0.706964 to 0.707691; av. 0.707212) comparing to the ambient seawater isotope signatures. The lack of reaction between basinal fluids and non-carbonate rocks is indicated by the negative δ26MgDol-DSM3 values in dolostones. However, initial dolomitization in the lower Arab is interpreted to have occurred under near-surface conditions, as supported by δ13CDolVPDB and δ26MgDol-DSM3 values.

This research presents new sights into: (1) multi-phase dolomitization evidenced by petrographic, mineralogical, and geochemical records of dolostones, (2) an archive of Mg isotopes of dolostones through geological times, as dolomite has been suggested to be a significant sink of seawater Mg isotope composition for global Mg cycle research, and (3) application of in-situ U-Pb dating of dolomite by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), for interpreting the origin of ancient dolomites.

Date of Award	12 Dec 2024
Original language	American English
Supervisor	Mohammad Alsuwaidi (Supervisor)