Rudy Swennen, K.U.Leuven (Belgium)
Jeff Lukasik, StatoilHydro (Norway)
David Hunt, StatoilHydro ASA (Norway)
Ian Sharp, StatoilHydro ASA (Norway)
Coarse crystalline hydrothermal dolomites make up major dolomite bodies in the Eldon Formation (Middle Cambrian) in British Columbia (Canada). Due to the retreat of the Daly Glacier (Yoho National Park), these dolomites are extremely well exposed and lithological variations were studied in horizontal and vertical sections. Cross-bedded grain- to packstone, bioturbated wackestone, laminated lagoonal mudstone and algal/microbial lithologies form shallowing-upward sequences that have been dolomitised. Two stages of dolomitisation can be differentiated, namely an early diagenetic dolomitisation that is likely to be related to circulation of (hyper)saline brines since evaporite pseudomorphs have been identified, and a post-burial stylolite dolomitisation, giving rise to coarse crystalline as well as zebra and cavernous dolomites. The latter have been studied in more detail.
The development of the coarse crystalline dolomite is clearly governed by the existence of a pre-existing calcite cemented fracture pattern. Dolomitisation preferentially develops along one side of these fractures. The transition between limestone and dolomite is often rather sharp if the limestones are dominated by peloidal mudstones/wackestones, but is more gradual in bioturbated and stylolitised limestones.
Matrix porosity in the dolomites is generally low, however regional variations in fracture porosity exist. The latter have been aerially mapped on a scale of several hundred of meters in two dimensions. Fractures cluster into "corridors", i.e. are grouped, with a clear increase in fracture pattern frequency towards a major fault zone. Fracture porosity is however low due to the infill by ferroan dolomite and a late stage of calcite which reflect different episodes in the reactivation of the fault system likely due to post-depositional orogenic processes.
Porous zebra, cavernous and very coarse crystalline dolomites have been encountered in the different vertical profiles studied. The latter preferentially developed below impermeable often non-dolomitised lime mudstone beds. In some of these (porous) dolomitised lithologies, disrupted stylolites are floating within the coarse crystalline matrix, pointing towards very high fluid pressures blasting apart the original host rock during dolomitisation. As a tentative working hypothesis, these features are linked to earth quake activity inducing focused overpressured fluid flow.