Clastic rocks

One of the most important areas of ancient rocks is the Isua Greenstone Belt (or supracrustal belt, as it is often called) (Appel et al., 1998), located in the northern part of the Isukasia terrane (Fig. 1.9). The Isua Greenstone Belt comprises a sequence of metabasalts, ultramafic rocks, clastic, and chemical sediments, with ages between 3.7 and 3.8 Ga (Nutman et al., 1996). The most ancient materials so far recorded from Isua are mineral... 

Reservoir rocks are either of clastic or carbonate composition. The former are composed of silicates, usually sandstone, the latter of biogenetically derived detritus, such as coral or shell fragments. There are some important differences between the two rock types which affect the quality of the reservoir and its interaction with fluids which flow through them. 

With a few exceptions reservoir rocks are sediments. The two main categories are siliciclastic rocks, usually referred to as elastics or sandstones , and carbonate rocks. Most reservoirs in the Gulf of Mexico and the North Sea are contained in a clastic depositional environment many of the giant fields of the Middle East are contained in carbonate rocks. Before looking at the significance of depositional environments for the production process let us investigate some of the main characteristics of both categories. 

It is believed that the majority of clastic reservoir rocks are water wet, but the subject of wettability is a contentious one. 

Primary porosity—porosily formed at the time the sediment was deposited. Sedimentary rocks that typically exhibit primary porosity are the clastic (also called fragmental or detrital) rocks, which are composed of erosional fragments from older beds. These particles are classified by grain size. 

The most stable minerals are often physically eroded before they have a chance to chemically decompose. Minerals that decompose contribute to the dissolved load in rivers, and their solid chemical-weathering products contribute to the secondary minerals in the solid load. The secondary minerals and the more stable primary minerals are the most important constituents of clastic sedimentary rocks. Consequently, the secondary minerals of one cycle of erosion are... 

Sedimentary rocks have formed as a result of accumulation and compaction of mineral particles derived from pre-existing rocks, transported from their original places of occurrence and deposited in new environments. The essential ingredients for the formation of sedimentary rocks are (i) source materials, (ii) mechanical and chemical disintegration of these source materials, (iii) transportation of the released materials either in a clastic form... 

Fig. 30.4. Changes in the volumes of minerals in the reservoir rock during the simulated alkali floods (Fig. 30.3) of a clastic petroleum reservoir using NaOH, Na2CC>3, and Na2SiC>3 solutions. Minerals that react in small volumes are omitted from the plots. Abbreviations Anal = analcime, Cc = calcite, Daw = dawsonite, Dol = dolomite, Kaol = kaolinite, Muse = muscovite, Parag = paragonite, Phlog = phlogopite, Qtz= quartz, Trid = tridymite.

Lithium isotope studies of sediments and sedimentary rocks have thus far concentrated on marine clastic and carbonate material. No systematic description of the effects of diagenetic processes on sediments has been made. Clay rich sediments are important to Li budgets in near-surface systems, as they concentrate Li relative to marine carbonates, which are among... 

Clastic sediments are reservoirs of information about weathering processes, but are sufficiently complex that no study has yet to realize their potential. Despite a number of initial reports of relatively isotopically heavy samples, the majority of data for clastic sedimentary rocks have an average 8 Li 0, equivalent to the estimated average isotopic composition of the continental crust. 

Clastic and pelagic sediments. Only a handful of clastic and pelagic samples have been analyzed for 5 Mo (Siebert et al. 2003). Clastic sediments (two samples) are indistinguishable from igneous rocks in their Mo isotope compositions. Pelagic clays (two samples) show enrichment in Mo (3.4 and 185 ppm), and also a shift toward lighter d Mo similar to ferromanganese sediments. 

Skelton, D., Clements, B., McCandless, T.E., Hood, C. Aulbach, S., Daview, R,. Boyer, L.P. 2003. The Buffalo Head Hills kimberlite province, Alberta. In Kjarsgaard, B.A. (ed.). Slave Province and Northern Alberta Field Trip Guidebook. Geological Survey of Canada, Miscellaneous Publication G-293. Skinner, E.M.W. Marsh, J.S. 2004. Distinct kimberlite pipe classes with contrasting eruption processes. Lithos, 76, 183-200. Sweet, A.R., Boyce, K,. Eccles, D.R. In prep. Palynological constraints on kimberlite emplacement models chronostratigraphy of host rock and clastic xenoliths, Buffalo Head Hills, Alberta. 

The Harvey Group consists of three units Harvey Mountain, Cherry Mountain, and York Mills Formations. The upper Harvey Mountain formation consists of felsic volcanic rocks, typically flow-banded massive rhyolitic lava accompanied by ash-fall tuff. Rocks of the middle Cherry Mountain formation are characterized by the predominance of ash-fall and ash-flow tuffs at the top and volcanogenic sedimentary rocks at the base. The lower York Mills formation is a sequence of interbedded clastic, volcanogenic and minor volcanic rocks, that latter consisting of felsic volcanic-clast conglomerate, volcanogenic sandstone, siltstone and shale, felsic crystal lithic lapilli tuff and flow-banded rhyolite. 

The Big Hole Brook Formation is the uppermost Formation in the Dalhousie Group and conformably overlies the Sunnyside Formation. The Big Hole Brook Formation consists of green-grey, micaceous, locally calcareous fine-grained thin- to thick-bedded sandstone and siltstone (Fig. 2). North of the deposit the Big Hole Brook Formation is unconformably overlain by clastic rocks of the Carboniferous Bonaventure Formation (Fig. 2). 

The Hickey and Hayes zones occur within a larger envelop of Zn-Pb-Ag-Fe sulfide mineralization that lies more or less along a north-south line straddling the contacts between the upper most flows and clastic rocks associated with the Archibald Settlement and bounding Sunnyside formations. Specifically, it is concentrated between and adjacent to subvertical, north-south striking faults that parallel the Black Point-Arleau Brook Fault. 

Trenches excavated across the veins are typically flooded by surface water however, three partially flooded trenches have exposed bedrock. From small (< 5m ) outcrops exposed in three of the trenches, it is apparent that the average grain size of the clastic sedimentary rocks and the abundance of coarse-grained detritus, increases from NE to SW away from the buried Caradocian shale, with arkosic sandstone most abundant in the SW near the Christopher vein. An extensive array of 98 drill holes provides excellent downhole, 3-D control on our understanding of the geology. 

In the northern part of the TCZ, the Chaleurs Group comprises subaerially deposited. Late Silurian bimodal volcanic rocks that overlie early Silurian shallow marine fine- to coarse-grained clastic and carbonate sedimentary rocks. The Chaleurs Group is overlain disconformably by bimodal volcanic rocks and interlayered sandstones and siltstones of the Dalhousie Group. 

Another application of stable isotopes in clastic rocks is the analysis of weathering profiles, which can potentially provide insight into the continental climate during... 

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