Microfacies analysis of the lower paleocene beda formation in Western Concession 59 and Block 59F, Sirte Basin, Libya /

The rock sequence of the Tertiary Beda Formation of S. W. concession 59 and 59F block in Sirte Basin of Libya has been subdivided into twelve platformal carbonate microfacies. These microfacies are dominated by muddy carbonates, such as skeletal mudstones, wackestones, and packstones with dolomites and anhydrite. Rock textures, faunal assemblages and sedimentary structures suggest shallow, clear, warm waters and low to moderate energy conditions within the depositional shelf environment. The Beda Formation represents a shallowing-upward sequence typical of lagoonal and tidal flat environments marked at the top by sabkha and brackish-water sediments. Microfossils include benthonic foraminifera, such as miliolids, Nummulites, - oerculina and other smaller benthonics, in addition to dasycladacean algae, ostracods, molluscs, echinoderms, bryozoans and charophytes. Fecal pellets and pelloids, along with the biotic allochems, contributed greatly to the composition of the various microfacies. Dolomite, where present, is finely crystalline and an early replacement product. Anhydrite occurs as nodular, chickenwire and massive textures indicating supratidal sabkha deposition. Compaction, micr it i zat ion , dolomit izat ion , recrystallization, cementation, and dissolution resulted in alteration and obliteration of primary sedimentary structures of the Beda Formation microfacies. The study area is located in the Gerad Trough which developed as a NE-SW trending extensional graben. The Gerad trough was characterized by deep-shallow water conditions throughout the deposition of the Beda Formation sediments. The study area is marked by several horsts and grabens; as a result of extent ional tectonism. The area was tectonically active throughout the Tertiary period. Primary porosity is intergranular and intragranular, and secondary processes are characterized by dissolution, intercrystalline, fracture and fenestral features. Diagenesis, through solution leaching and dolomitization, contributed greatly to porosity development. Reservoir traps of the Beda Formation are characterized by normal fault blocks and the general reservoir characteristics/properties appear to be facies controlled.

Physical volcanology, sedimentology, stratigraphy and petrochemistry of the Berry Creek metavolcanics: an Archean calc-alkaline complex, Lake of the Woods, Ontario

The steeply dipping, isoclinally folded early Precambrian (Archean) Berry Creek Metavolcanic Complex comprises primary to resedimented pyroclastic, epiclastic and autoclastic deposits. Tephra erupted from central volcanic edifices was dumped by mass flow mechanisms into peripheral volcanosedimentary depressions. Sedimentation has been essentially contemporaneous with eruption and transport of tephra. The monolithic to heterolithic tuffaceous horizons are interpreted as subaerial to subaqueous pumice and ash flows, secondary debris flows, lahars, slump deposits and turbidites. Monolithic debris flows, derived from crumble breccia and dcme talus, formed during downslope collapse and subsequent gravity flowage. Heterolithic tuff, lahars and lava flow morphologies suggest at least temporary emergence of the edifice. Local collapse may have accompanied pyroclastic volcanism. The tephra, produced by hydromagmatic to magmatic eruptions, were rapidly transported, by primary and secondary mechanisms, to a shallow littoral to deep water subaqueous fan developed upon the subjacent mafic metavolcanic platform. Deposition resulted from traction, traction carpet, and suspension sedimentation from laminar to turbulent flows. Facies mapping revealed proximal (channel to overbank) to distal facies epiclastics (greywackes, argillite) intercalated with proximal vent to medial fan facies crystal rich ash flows, debris flows, bedded tuff and shallow water to deep water lava flows. Framework and matrix support debris flows exhibit a variety of subaqueous sedimentary structures, e.g., coarse tail grading, double grading, inverse to normal grading, graded stratified pebbly horizons, erosional channels. Pelitic to psammitic AE turbidites also contain primary stru~tures, e.g., flames, load casts, dewatering pipes. Despite low to intermediate pressure greenschist to amphibolite grade metamorphism and variably penetrative deformation, relicts of pumice fragments and shards were recognized as recrystallized quartzofeldspathic pseudomorphs. The mafic to felsic metavolcanics and metasediments contain blasts of hornblende, actinolite, garnet, pistacitic epidote, staurolite, albitic plagioclase, and rarely andalusite and cordierite. The mafic metavolcanics (Adams River Bay, Black River, Kenu Lake, Lobstick Bay, Snake Bay) display _holeiitic trends with komatiitic affinities. Chemical variations are consistent with high level fractionation of olivine, plagioclase, amphibole, and later magnetite from a parental komatiite. The intermediate to felsic (64-74% Si02) metavolcanics generally exhibit calc-alkaline trends. The compositional discontinuity, defined by major and trace element diversity, can be explained by a mechanism involving two different magma sources. Application of fractionation series models are inconsistent with the observed data. The tholeiitic basalts and basaltic andesites are probably derived by low pressure fractionation of a depleted (high degree of partial melting) mantle source. The depleted (low Y, Zr) calc-alkaline metavolcanics may be produced by partial melting of a geochemically evolved source, e.g., tonalitetrondhjemite, garnet amphibolite or hydrous basalt.