Fault-controlled dolomitization in a rift basin

Abstract

There are numerous examples of fault-controlled, so-called hydrothermal dolomite (HTD), many of which host economic mineral deposits or hydrocarbons, but there remains a lack of consensus as to how they form. In particular, multiple phases of diagenetic overprinting can obscure geochemical fingerprints. Study of a Cenozoic succession with a relatively simple burial history here provides new insights into the development of differentially dolomitized beds. The Hammam Faraun fault (HFF) block within the Suez Rift, Egypt, hosts both massive and stratabound dolostone bodies. Non-fabric-selective massive dolostone is limited to the damage zone of the fault, while fabric-selective stratabound dolostone bodies penetrate nearly 2 km into the footwall. Oligo-Miocene seawater is interpreted to have been drawn down discrete faults into a deep aquifer and convected upwards along the HFF. Escape of fluids from the incipient HFF into the lower Thebes Formation led to differential, stratabound dolomitization. Once the HFF breached the surface, fluid circulation focused along the fault plane to form younger, massive dolostone bodies. This study provides a snapshot of dolomitization during the earliest phases of extension, unobscured by subsequent recrystallization and geochemical modification. Contrary to many models, stratabound dolomitization preceded non-stratabound dolomitization. Fluids were hydrothermal, but with little evidence for rapid cooling and brecciation common to many HTD bodies. These results suggest that many of the features used to interpret and predict the geometry of HTD in the subsurface form during later phases of structural deformation, perhaps overprinting less structurally complex dolomite bodies.

Controls on the formation of stratabound dolostone bodies, Hammam Faraun Fault block, Gulf of Suez

“…The origin of the massive dolostone bodies in the study area are further discussed in Hollis et al . ().…”

“… Stratabound dolostone bodies, varying in length from 5 m up to 300 m along depositional dip, and ranging in thickness between 25 cm and 5 m. These dolostone bodies extend for up to 2 km away from the HFF, before disappearing beneath the sedimentary cover. Massive dolostone bodies, up to 500 m wide (distance from the HFF) and 60 to 80 m thick. Dolostone tongues associated with these massive bodies extend for up to 100 m away from the massive dolostone bodies, forming 1 to 5 m thick bands (Hirani, ; Hollis et al ., ). …”

“…Hollis et al . () demonstrated that in this well‐exposed example, the stratabound dolostone pre‐dates, and is not genetically related to, cross‐cutting non‐stratabound dolostone. This is contrary to the interpretation of many dolostone bodies formed in proximity to faults.…”

Fault‐controlled dolomite bodies as palaeotectonic indicators and geofluid reservoirs: New insights from Gargano Promontory outcrops

“…In addition, other mechanisms could have produced a weak thermal gradient, such as a little geothermal anomaly related to the transtensional tectonic setting that was active in Gargano Promontory at the time of dolomitization. In such a tectonic setting, dolomitization models that rely on geothermal convection of former seawater circulating along faults (Corbella et al ., ; Hollis et al ., ) can explain the dolomitization in Gargano Promontory; especially the fault‐plane convection model proposed by Hollis et al . ().…”

“…In such a tectonic setting, dolomitization models that rely on geothermal convection of former seawater circulating along faults (Corbella et al ., ; Hollis et al ., ) can explain the dolomitization in Gargano Promontory; especially the fault‐plane convection model proposed by Hollis et al . (). This model relies on seawater that descends down discrete, surface‐breaching faults (Fig.…”