Structural controls on non fabric‐selective dolomitization within rift‐related basin‐bounding normal fault systems: Insights from the Hammam Faraun Fault, Gulf of Suez, Egypt

Abstract

Fault-controlled dolostone bodies have been described as potential hydrocarbonbearing reservoirs. Numerous case studies have described the shape and size of these often non fabric selective dolostone bodies within the vicinity of crustal-scale lineaments, usually from Palaeozoic or Mesozoic carbonate platforms, which have undergone one or more phases of burial and exhumation. There has been little attention paid, however, to fault-strike variability in dolostone distribution or the preferential localization of these bodies on particular faults. This study focuses on dolostone bodies adjacent to the Hammam Faraun Fault (HFF), Gulf of Suez. This crustalscale normal fault was activated in the Late Oligocene, coincident with the onset of extension within the Suez Rift. Dolomitization in the prerift Eocene Thebes Formation occurred in the immediate footwall of the HFF forming two massive, non facies selective dolostone bodies, ca. 500 m wide. Facies-controlled tongues of dolostone on the margins of the massive dolostone bodies extend for up to 100 m. The geochemical signature of the dolostone bodies is consistent with replacement by Miocene seawater, contemporaneous with the rift climax and localization of strain along the HFF. A conceptual model of dolomitization from seawater that circulated within the HFF during the rift climax is presented. Seawater was either directly drawn down the HFF or circulated from the hanging wall basin via a permeable aquifer towards the HFF. The lateral extent of the massive dolostone bodies was controlled by pre-existing HFF-parallel fracture corridors on the outer margins of the damage zone of the fault. The behaviour of these fracture corridors alternated between acting as barriers to fluid flow before rupture and acting as flow conduits during or after rupture. Multiple phases of dolomitization and recrystallization during the ca.10 Ma period in which dolomitization occurred led to mottled petrographical textures and wide-ranging isotopic signatures. The localization of dolomitization on the HFF is interpreted to reflect its proximity to a rift accommodation zone which facilitated vertical fluid flow due to perturbed and enhanced stresses during fault interaction. It is possible that the presence of jogs along the strike of the fault further focused fluid flux. As such, it is suggested that the massive dolostones described in --

Understanding controls on hydrothermal dolomitisation: insights from 3D reactive transport modelling of geothermal convection

“…Finally, dolomitisation associated with the hydrothermal and geothermal circulation of seawater has been suggested as an important but poorly constrained sink for Mg in seawater (Shalev et al, 2019). Thus, these processes have fundamental implications for understanding the global budgets of Mg, Ca, and C, which are linked by the common con-trols of weathering, volcanism, and carbonate precipitation (Arvidson et al, 2011;Elderfield, 2010;Holland, 2005). Combining recent magnesium isotope ( 26 Mg) data for lowtemperature hydrothermal fluids with evidence of a constant 26 Mg over the past 20 Myr, Shalev et al (2019) suggest that there is a significant unexplained flux of Mg from the modern ocean (estimated at 1.5-2.9 Tmol yr −1 ).…”

Understanding controls on hydrothermal dolomitisation: insights from 3D Reactive Transport Modelling of geothermal convection

“…Our model simulates a single event lasting a few tens of kyr within a fault that retains its transmissivity, but many outcrop studies suggest multiple episodes of fluids flow, each with the potential to recrystallize the previous dolomite. For example, in the Hammam Faraun example, Sr-isotope dating suggests that the massive dolomite formed over a 10 Ma period but likely within multiple episodes during which fault zone permeability permitted more active circulation (Hirani et al, 2018;Hollis et al, 2017). Fault transmissivity is dynamic, and movement of large quantities of fluids within faults during earthquakes via "seismic pumping" has been linked to submarine hydrothermal mineralisation that is episodic on time scales ranging from a minutes to tens of thousands of years and controlled by volcanic and tectonic processes (Brantut, 2020;Sibson, 2001;Sibson et al, 1975).…”

“…Even relatively minor evaporative concentration of the overlying seawater will significantly lower the temperature of dolomitisation (Al-Helal et al, 2012;. Thus, clumped isotope analyses suggesting temperatures <100 °C for the massive dolostone developed in the Hammam Faraun Fault (Hirani et al, 2018) could suggest formation from mesohaline seawater; not unreasonable as dolomitisation has been dated to the Miocene, when the Gulf of Suez was undergoing desiccation and evaporite precipitation (Hollis et al, 2017). Temperatures predicted from our simple models are more similar to those reported for fault-related dolomites in the Ranero area, northern Spain (120-200 °C;Shah et al, 2010) interpreted to form during multiphase dolomitisation, potentially at variable salinities up to 22 wt.% NaCl (López-Horgue et al, 2010).…”

“…Faraun Fault described byHollis et al (2017) andHirani et al (2018) shows marked geometric similarities. The dolostone bodies are constrained to the FDZ of the Hammam Faraun Fault, postdating earlier stratabound dolostones within Eocene limestone country rock.…”

“…An increasing number of studies have suggested fault-related dolomitisation by seawater or seawater-derived fluids (Carmichael and Ferry, 2008;Corbella et al, 2014;Haeri-Ardakani et al, 2013;Hollis et al, 2017;Wilson et al, 2007). Thus, for example in the Hammam Faraun fault (HFF) area, Egypt, strontium isotope and REE signatures suggest that massive dolostone bodies limited to the damage zone of the fault formed from Oligo-Miocene seawater during development of the Suez Rift (Hirani et al, 2018;Hollis et al, 2017). We have previously suggested a conceptual model for formation of these dolostones whereby Mg 2+ -rich seawater could have been drawn down into a convection cell along the fault, heated and ascend to form massive dolostone (Hollis et al, 2017).…”