Dolomitization – Question C
One of the conundrums of carbonate diagenesis is that seawater is supersaturated in dolomite, yet dolomite is rarely directly precipitated in the present day. Discuss the strengths and weaknesses of different mechanisms that have been postulated to explain dolomitization, and why it is important to understand from a hydrocarbon perspective.
Introduction
Dolomite (CaMg〖(CO_3)〗_2) is found in carbonate rocks of all ages that formed under a variety of conditions, but today it only forms in hypersaline lagoons, lakes and tidal flats. This suggests that most dolomite is secondary in origin as a result of dolomitization: the replacement of calcite or aragonite (CaCO_3) by dolomite soon after deposition or during diagenesis, generally increasing porosity. Dolomitization is therefore highly important in sedimentology and in hydrocarbon exploration but it is still not fully understood. A number of models have been proposed to explain its occurrence and why it doesn’t form everywhere, even though seawater is supersaturated with respect to dolomite (Nichols, 1999; Tucker, 2001).
Dolomitization Models
Due to dolomite’s highly ordered structure there are a number of factors that inhibit its precipitation: high ionic strength of seawater, Mg2+ hydration and low CO3- activity, even under supersaturated conditions and so certain conditions are required. These are: a sufficient supply of Mg2+ that is long lasting, an initial porosity, pressure differential for pore fluid migration and a pore fluid undersaturated in Ca2+. Therefore it is common that before dolomitization takes place the Mg:Ca ratio must increase, forcing dolomite out of solution. If these conditions are not met there will be no dolomitization and inste...
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The shelf-edge includes carbonate-to-clastic facies transition and tectonic uplift and erosion of the carbonates followed by deposition of the clastics. The Saint Peter Sandstone is a well-sorted, almost pure quartz arenite deposited during a major mid-Ordovician low stand. Clastics spread across an exposed carbonate platform by transportation. This is shown by the well-rounded, frosted texture of the quartz grains.
This is a report based on three days of observations and testing in the region known as the Peterborough drumlin field. It will address a variety of regional elements, such as climate, soil, vegetation, hydrology, geomorphology, and geology. A variety of sites located on the Canadian Shield, the zone of thick glacial deposits to the south, and the transition between them will be the focus of the report. It is supplemented with previous research on the region. September 8, 1999, day one of the field study involved an area of largely granite bedrock that is part of the Canadian Shield and is the most northern point of study (see Map 2). September 9, 1999, day two, involved three main areas of study: the Bridgenorth esker (Map 3), Mark S. Burnham Park (Map 4), and the Rice Lake drumlin (Map 6). These sites are in areas of thick glacial deposits. September 10, 1999, day three, involved studying the Warsaw Caves (see Map 5) as a transition zone between Precambrian Shield rock to the north and Paleozoic rock to the south. A general map of the entire study region is provided by Map 1.
The central belt of the Franciscan Complex represents older and more metamorphosed units of rock best characterized as a melange. Blocks of graywacke, greenstone, chert, limestone, and blueschists are sheared and thrust upon one another in a choatic mix (Isozaki and Blake, 1994). In contrast to the coastal belt, metamorphism is higher in grade here and dominated by pumpellyite which formed within the matrix of graywacke (Hagstrum and Murchey, 1993). The mixing of these units makes a stratigraphic subdivision difficult but analysis of the graywacke slabs indicates that the depositional environment was also deep sea, near to the continent. Turbidity currents in this environment deposited much of the sediment in both the coastal and central belts.
This sedimentary rock has hardened over the many years with sand shells, small pebbles, grains of sand and rocks of various sizes. In comparison to our 4.5 billion year old Earth, these sand shells might as well be brand new, when in reality they could be up to 1,000 years old. If the sandstone were to be replaced with calcite it would completely change the subclass of rock, it would then be chemical & organic limestone. The variation in sand stone is due to different rates of deposition and change in patterns of the sediment movement (Mc Knight, p. 384). These tightly compacted varying stones and shells will be weathered away by wind and waves over time and could eventually be reduced to a rock the size of your hand.
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The Burgess Shale Fauna is a fauna that was constructed based on a group of fossils that were initially found, in the Burgess Shale area in the Canadian Rockies (Gould, 1989). They are a very important group of fossils as “modern multicellular animals make their first unprotected appearance in the fossil record some 570 million years ago” through this group (Gould, 1989, pp. 24). Moreover the Burgess Shales are known to have preserved the soft parts of animals enabling us to get a better understanding of life at the time.
Minerals play an important role in our day-to-day life but we often not contemplate how the minerals are obtained. Minerals are scattered all over the world just like any other resources. Due to the natural processes of magma flow, hydrothermal gradients, sedimentation, and evaporation, Minerals are concentrated in various areas of the Earth’s crust. Obtaining these minerals for human use involves four general steps:
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The Burgess Shale is located in British Columbia’s Yoho National Park; Part of the ancient landmass called Laurentia (Scott, et al., 2000). Fossils found within the formation dating back 545-525 million years ago represent original species from the Cambrian explosion, a relativel...
inferred for the reservoir (4). The magma ascent to the surface occurred through a conduit of possibly 70 to 100 m in diameter (5). A thermal model predicts that such a reservoir should contain a core of partially molten magma (6) that can be detected by high-resolution seismic tomography.