Ferroan dolomite overgrowths

Fig. 5. Authigenic ferroan dolomite overgrowth (o) on a core of fractured detrital dolomite (d) from the Bismantova Formation. Overgrowth development preceded precipitation of calcite cement. Similar dolomite overgrowths are observed in the Borello Formation. Back-scattered electron image.

Fig. 7. Thin-section scale localization of authigenic ferroan dolomite. Back-scattered electron images. Scale bars 100 tm. (A) Ferroan dolomite (f) localized on non-ferroan detrital dolomite (d) that has been fractured, partially dissolved, and partially replaced by the authigenic overgrowth. Note that the outer zones of the ferroan dolomite (f ) are brighter, reflecting their greater Fe content. (B) Ferroan dolomite (ankerite) (f) localized in the vicinity of a partially dissolved and replaced K-feldspar (k). The arrow indicates euhedral termination on a quartz overgrowth.

Dolomite, ferroan dolomite and ankerite occur mostly as thin (< 20 pm) overgrowths on detrital dolomite grains (Figs 5A,B,F and 6A) or, rarely, as... 

Volumetrically minor authigenic ferroan dolomite is present locally as overgrowths on distinctive cores of partially dissolved detrital dolomite (Fig. 5). Prominent zoning of Fe and Mg in these overgrowths is readily observed on back-scattered electron images. Dolomite precipitation clearly precedes the formation of calcite cement. 

Early ferroan dolomite occurs in an 18m thick fine-grained unit of the Hibernia Sandstone in well B-27, where in thin zones (<1 m thick) it occluded all available pore space and replaced detrital quartz. The early diagenetic nature of this cement (crystal size 30-200 pm, average 50 pm) is indicated by a high minus-cement porosity (>30%) and the lack of quartz overgrowths or other cements. 

Porosity reduction due to recementation af ter the dissolution event is generally small (1-2%) and involved kaolinite, quartz, pyrite and ferroan cal-cite and dolomite cements. Locally, as much as 10% of late cements have been precipitated. Kaolinite usually reaches no more than trace amounts, although locally up to 6% has been observed. Late quartz overgrowths are negligible. Late pyrite generally does not exceed 1% however, locally it may occlude all available pore space. Late ferroan calcite cement has completely filled the available pores at a few levels, but the total porosity loss by this cement and by late ferroan dolomite is not significant. 

Available drill cores from the 0-35 well show very low thin-section porosity (<5%). Advanced mechanical compaction, including shale-clast deformation, together with ferroan dolomite cement and quartz overgrowths, has occluded most of the porosity. 

---