Dissolution of ooids in seawater‐derived fluids – an example from Lower Permian re‐sedimented carbonates, West Texas, USA

Abstract

Selective dissolution of metastable minerals and certain pore types, such as moldic and oomoldic pores, are widely accepted to indicate diagenesis by meteoric fluids in carbonate rocks. In this study, oomoldic-rich re-sedimented carbonates from the Happy Field in the Midland Basin, which contain no evidence of subaerial environments, indicate that such pores may form in other settings. Ooid dissolution in the reservoir was evaluated through petrography, scanning electron microscopy, stable isotopes (d 18 O and d 13 C), elemental geochemistry, and X-ray diffraction of bioclasts, ooids and associated cements. Cements are primarily dogtooth and equant low-magnesium calcite, poikilotopic celestine is present, and late-stage low-magnesium calcite occurs in lesser amounts. Equant low magnesium calcite cements are interpreted to reflect the initial stages of pore filling, synchronous with aragonitic ooid dissolution, thus implying that oomoldic pores and equant cements formed in similar fluids. Although the prominent equant low magnesium calcite cement textures mimic those often associated with meteoric fluids, geochemistry suggests a marine origin. The equant cements have d 18 O (Vienna Pee Dee Belemnite) ratios between À2.0& and À3.5&, and d 13 C (Vienna Pee Dee Belemnite) ratios between 4& and 5&. Magnesium/calcium ratios show values, ranging from 5 to 142 mmol/mol (n = 223, average 23.2 and SD = 20.1). Strontium/calcium ratios present values from 0.3 to 2.6 mmol/mol (n = 223, average 0.71 mmol/mol, SD = 0.35). Geochemical data coincide with typical lowlatitude Permian marine carbonate values and further signify diagenesis within the marine phreatic realm. To explain this process, the results indicate that the dissolution of aragonite ooids and precipitation of early cements occurred in environments ranging between the seafloor to shallow burial dominated by undersaturated marine fluids. Undersaturation was triggered by organic matter decomposition occurring at the seafloor and in the shallow subsurface immediately below. These findings have broader implications for marine and burial carbonate diagenesis as well as diagenesis of re-sedimented deposits.

The Steptoean Positive Carbon Isotope Excursion (SPICE), inorganic aragonite precipitation and sea water chemistry: Insights from the Middle–Late Cambrian Port au Port Group, Newfoundland

“…In oxidising environments (e.g. surface or shallow burial), the decomposition of organic matter through processes such as sulphate oxidation or nitrification, can result in dissolution of CaCO 3 (Diaz et al, 2015; Laya et al, 2021). It is unlikely that the PJF would have been affected by fluids rich in oxygen during its early diagenesis.…”

A Redox‐Based Model for Carbonate Platform Drowning and Ocean Anoxic Events

“…In fact, modern oceans produce carbonate minerals at a rate that is four times higher than weathering inputs (Broecker & Peng, 1982; Sarmiento & Gruber, 2006; Sigman & Boyle, 2000). This “overproduction” of carbonate is compensated by carbonate dissolution within deep ocean basins and shallow oxic porewaters more broadly (Broecker, 1998; Laya et al., 2021; Melim et al., 2002; Walter & Burton, 1990): …”

“…Carbonate dissolution in the modern ocean is governed by several factors. Some carbonate dissolves at depth because carbonate minerals are more soluble at low temperatures and high pressures (Zeebe & Wolf‐Gladrow, 2001), but this inorganic effect is comparatively small and a substantial amount of dissolution is driven by aerobic respiration of organic matter, both in the water column and within sedimentary pore fluids (Archer, 1996; Dunne et al., 2012; Hales, 2003; Laya et al., 2021). Biologically driven dissolution is widespread in the modern oceans because oxygenated bottom waters overlie >95% of the seafloor (Helly & Levin, 2004).…”