A depositional model for the terminal Neoproterozoic–Early Cambrian Ara Group evaporites in south Oman

Abstract

Six evaporite–carbonate sequences are recognized in the terminal Neoproterozoic–Early Cambrian Ara Group in the subsurface of Oman. Individual sequences consist of a lower, evaporitic part that formed mainly during a lowstand systems tract. Overlying platform carbonates contain minor amounts of evaporites and represent transgressive and highstand systems tracts. Detailed sedimentological and geochemical investigation of the evaporites allowed reconstruction of the depositional environment, source of brines and basin evolution. At the beginning of the evaporative phase (prograding succession), a shallow‐water carbonate ramp gradually evolved into a series of shallow sulphate and halite salinas. Minor amounts of highly soluble salts locally record the last stage of basin desiccation. This gradual increase in salinity contrasts sharply with the ensuing retrograding succession in which two corrosion surfaces separate shallow‐water halite from shallow‐water sulphate, and shallow‐water sulphate from relatively deeper water carbonate respectively. These surfaces record repeated flooding of the basin, dissolution of evaporites and stepwise reduction in salinity. Final flooding led to submergence of the basin and the establishment of an open‐water carbonate ramp. Marine fossils in carbonates and bromine geochemistry of halite indicate a dominantly marine origin for the brines. The Ara Group sequences represent a time of relatively stable arid climate in a tectonically active basin. Strong subsidence allowed accommodation of evaporites with a cumulative thickness of several kilometres, while tectonic barriers simultaneously provided the required restricted conditions. Subsidence allowed evaporites to blanket basinal and platform areas. The study suggests a deep‐basin/shallow‐water model for the evaporites.

Geology and petrology of the Hormuz dolomite, Infra-Cambrian: Implications for the formation of the salt-cored Halul and Shraouh islands, Offshore, State of Qatar

“…Sulfide mineralization is common in cap rocks of salt domes trapping hydrocarbons (Schröder et al, 2003). Hydrothermal mineralization (e.g., well-crystalline hematite, galena, pyrite, and fluorite) is common within the Hormuz series (Ala, 1974;Salah, 1996;Schrö der et al, 2003). The strong correlation of Fe, Ni, Cu, and Zn might suggest the input of a mineralizing fluid from some deep source, or associated with rift-related igneous activity in the Hormuz series .…”

A paired sulfate–pyrite δ34S approach to understanding the evolution of the Ediacaran–Cambrian sulfur cycle

“…In strata beneath the modern day South Oman Salt Basin (SOSB), the Ara Group exists as a series of six carbonate-evaporite shallowing-upward cycles (Fig. 3), deposited in a strongly subsiding block-faulted basin (Grotzinger et al, 2002;Amthor et al, 2003;Schrö der et al, 2003b). The basal Ara unit (A0) is believed to have been deposited prior to the onset of periodic basin restriction based on the absence of evaporites and/or evaporitic enrichment in e.g., d 13 C carb /d 18 O carb (Amthor et al, 2003).…”

“…3b, Schrö der et al, 2004), which have been shown to originate from marine waters (Horita et al, 2002;Schrö der et al, 2003a;Brennan et al, 2004). Despite the evaporitic conditions associated with primary evaporite deposition, stratiform anhydrite is believed to accurately record the isotopic composition of seawater sulfate because of nearly quantitative removal of seawater sulfate and the small ($1.6&) isotopic fractionation associated with gypsum/anhydrite deposition (Raab and Spiro, 1991).…”

Depositional environments and facies of the Late Triassic Abu Ruweis Formation, Jordan

“…High rates of accumulation of organic matter indicate that bottom waters were anoxic (Kirkland, 2003). The development of a barrier caused restriction of the basin, associated with relatively stable arid climatic conditions, allowing deposition of a thick prograding succession, similar to that described by Schroder et al (2003) in the Early Cambrian Ara Group in Oman. Kendall (1992) indicated that low-lying basins with a flat basin floor are flooded rapidly leading to rapid establishment of open-marine conditions.…”