Stratigraphic architecture and fracture-controlled dolomitization of the Cretaceous Khami and Bangestan groups: an outcrop case study, Zagros Mountains, Iran

Abstract

The Barremian–Aptian upper Khami Group and Albian–Campanian Bangestan Group have been studied at outcrop in Lurestan, SW Iran. The upper Khami Group comprises a thin deltaic wedge (Gadvan Fm) transgressively overlain by shelfal carbonates (Dariyan Fm). The Dariyan Fm can be divided into lower and upper units separated by a major intra-Aptian fracture-controlled karst. The top of the Daryian Fm is capped by the Arabian plate-wide Aptian–Albian unconformity. The overlying Bangestan Group includes the Kazhdumi, Sarvak, Surgah and Ilam formations. The Kazhdumi Fm represents a mixed carbonate-clastic intrashelf basin succession, and passes laterally (towards the NE) into a low-angle Orbitolina-dominated muddy carbonate ramp/shoal (Mauddud Mbr). The Mauddud Mbr is capped by an angular unconformity and karst of latest Albian–earliest Cenomanian age. The overlying Sarvak Fm comprises both low-angle ramp and steeper dipping (5–10°) carbonate shelf/platform systems. Three regionally extensive karst surfaces are developed in the latest Cenomanian–Turonian interval of the Sarvak Fm, and are interpreted to be related to flexure of the Arabian plate margin due to the initiation of intra-oceanic deformation. The Surgah and Ilam Fm represent clastic and muddy carbonate ramp depositional systems respectively.Both The Khami and Bangestan groups have been affected by spectacularly exposed fracture-controlled dolomitization. Dolomite bodies are 100 m to several km in width, have plume-like geometry, with both fracture (fault/joint) and gradational diagenetic contacts with undolomitized country rock. Sheets of dolomite extend away from dolomite bodies along steeply dipping fault/joint zones, and as strata-bound bodies preferentially following specific depositional/diagenetic facies or stratal surfaces. There is a close link between primary depositional architecture/facies and secondary dolomitization. Vertical barriers to dolomitization are low permeability mudstones, below which dolomitizing fluids moved laterally. Where these barriers are cut by faults and fracture corridors, dolomitization can be observed to have advanced upwards, indicating that faults and joints were fluid migration conduits.Comparisons to Jurassic–Cenozoic dolomites elsewhere in Iran, Palaeozoic dolomites of North America and Neogene dolomites of the Gulf of Suez indicate striking textural, paragenetic and outcrop-scale similarities. These data imply a common fracture-controlled dolomitization process is applicable regardless of tectonic setting (compressional, transtensional and extensional).

Reactive transport modelling of hydrothermal dolomitisation using the CSMP++GEM coupled code: Effects of temperature and geological heterogeneity

“…The dolomitisation patterns presented in section 3.4 resemble those from a number of different outcrops, including the Zagros Mountains in Iran, the Canadian Rocky Mountains, and the Basque-Cantabrian Basin in the North of Spain, where dolomite geochemistry indicates formation from high temperature fluids (Sharp et al, 2010;Lopez-Horgue et al, 2010;Dewit et al, 2014). Although a detailed stratigraphic description of the dolomitised outcrops above Lake O'Hare in the Canadian Rocky Mountains ( fig.…”

“…These models all produce dolomite bodies with geometries comparable to those of hydrothermal dolomite bodies observed at several outcrops (Corbella et al, 2014;Sharp et al, 2010;Lopez-Horgue et al, 2010;Dewit et al, 2014). Four models are presented, which provide insight into the influence of fault geometry and permeability of fault zone and of boundary conditions on the patterns of hydrothermal dolomitisation.…”

Linking process, dimension, texture, and geochemistry in dolomite geobodies: A case study from Wadi Mistal (northern Oman)

“…Hydrothermal dolomite often occurs in association with thrust and normal faults (Sharp et al, 2010) as well as basement-rooted wrench faults (Smith, 2006). Such dolomitization could be linked to increased heat flow associated with salt domes (Beavington-Penney et al, 2008) or to convective flow caused by igneous activity whether or not combined with tectonically induced flow related to fault reactivation (Wilson et al, 2007).…”

“…However, dolomite bodies formed by other dolomitization mechanisms, such as structurally-controlled dolomite bodies (e.g. Davies and Smith, 2006;Lonnee and Machel, 2006;López-Horgue et al, 2010;Shah et al, 2010;Sharp et al, 2010), are not expected to have a close tie with the depositional environment, especially when the host rock is significantly compacted at the time of dolomitization. As a consequence, their dimension and spatial distribution cannot be predicted in subsurface based on stratigraphic principles alone.…”

Sarvak Reservoir Facies Characteristics in One of the South West Field in Iran

“…The Sarvak Formation includes mostly carbonate in lithology and was composed of sequence of thin to medium-bedded limestone and massive limestone [1]. This formation predominantly characte-rized by cyclic stacks of shallow-marine carbonates and has recently been investigated in detail as a reservoir rock [4] [5]. The Sarvak Formation attains about 800 m thin, and medium to thick carbonate layers in thickness in its type section (Bangestan Mountain in vicinity of Izeh area).…”