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Primary Silica Precipitation Prior to the Evolution of Siliceous Organisms, an Example from the Precambrian-Cambrian Athel Silicilyte, South Oman Salt Basin

Ibrahim M. Al Rajaibi¹, Cathy Hollis², and Joe H. Macquaker³
¹Petroleum Development of Oman, Muscat, Oman
²School of Earth, Atmospheric and Environmental Sciences, University of Manchester, UK
³ExxonMobil, Houston, TX, USA

Prior to the evolution of siliceous organisms (diatoms, siliceous sponges and radiolaria) in the Proterozoic, chert formation is believed to have been largely controlled by chemical and/or biochemical processes during shallow to deep burial diagenesis. Cherts found in the upper Proterozoic (<1.8 Ga; after cessation of Banded Iron Formations deposition) are principally nodular or form discontinuous thin beds (few centimetres) that are precipitated by replacement of carbonate minerals, permineralisation of organic matter and/or by filling pore spaces and cavities. Their main locus of precipitation was in carbonate facies deposited in peritidal environments and their volumes are insignificant relative to the host rocks.

Conversely, the upper Proterozoic Athel Silicilyte in the South Oman Salt Basin is unique in term of its large thickness (390 m), homogeneity and depositional environment. It is also a prolific reservoir in Oman with high porosity (up to 34 %) and high oil saturation (80 %) typically encased in salt at a depth of 4-5 km. It has been mapped within the Athel Basin, interpreted as a fault-bounded rift basin surrounded by shallow water carbonates of the Ara Formation.

Detailed petrographical and inorganic geochemical analyses were carried out on 107 core samples of the Athel Silicilyte to determine its origin. The analyses reveal that the Athel Silicilyte is dominated by microbially laminated microcrystalline quartz (average = 80 wt. %; 1-5 μm), and minor amounts of detrital material, pyrite and organic matter. The microcrystalline quartz crystallography (non luminescent euhedral-subhedral) and the absence of any shallow water structures suggest that that quartz precipitated in situ below storm-wave influence. The homogeneity of the unit across the Athel Basin, absence of any pseudomorphs, its thickness, loose packing of detrital grains and association with syngentic framboidal pyrites all suggest that the Athel Silicilyte is a primary precipitate. There is no evidence for any replacement of precursor minerals or lithologies. The source of dissolved silica in the Athel Silicilyte is interpreted to be the silica-rich Precambrian seawater, prior to the Cambrian explosion and the evolution of silica-secreting organisms. This interpretation is strongly supported by rare earth element (REE) profiles, which exhibit a strong anoxic seawater signature. They also rule out the possibly of Athel being a hydrothermal precipitate.

The strong enrichments of redox-sensitive elements (U, V and Mo), Mn-depletion, and framboidal pyrites size-distributions (3.6-4.0 μm), all suggest that the water column was euxinic during precipitation of the Athel Silicilyte, allowing dissolved silica to form hydrogen bonds with organic matter. Consequently, silica became supersaturated in and on organic matter tissues leading to silica nucleation and polymerisation. Similar chert formations could be found in Proterozoic strata in settings that were organic matter-rich, detrital-starved and euxinic (possibly the Liuchapo Formation in south China).

AAPG Search and Discovery Article #90175©2013 AAPG Hedberg Conference, Beijing, China, April 21-24, 2013