The Simple Ideal Model For Understanding Sedimentary Rocks The key to understanding sedimentary rocks is to realize that all sedimentary processes of weathering, transportation, and deposition are aimed at one goal - reaching the three final end products of all sedimentary processes, quartz sand, shale (clay), and limestone (CaCO3).
How Sedimentary Rocks Are Formed
How Sedimentary Rocks Are Made Imagine an average continental igneous rock, a granodiorite, as at right (click picture to enlarge). It contains quartz, and feldspar, and mafic minerals, a representative sampling of the eight rock forming minerals in an igneous rock. Now imagine we are going to do every sedimentary process to that rock that it is possible to do, including complete weathering, and complete transportation, sorting and deposition. The results are always the same - quartz sandstone, shale, and limestone separated from each other in different depositional environments: the beach, near shelf, and far self.eight rock forming minerals Sea diagram previous slide
Process of Creating Sedimentary Rocks Sedimentary systems work this way because of two processes. WEATHERING: Weathering is the breakdown of one mineral/rock into another. Of the eight igneous rock forming minerals all are subject to degradation (weathering into something else), except quartz. Quartz, for all intents and purposes, does not weather and will survive in the system relatively unscathed. The remaining seven rock forming minerals all dissolve to make the sea salty, or decompose to form new minerals stable at the earth's surface. Orthoclase, for example, breaks down to form clay, and the calcium in Ca plagioclase goes into solution to form CaCO3. There are lots of other weathering products, of course, but they are just details. The simple, ideal model predicts three end products, quartz sand, shale, and limestone, which all together compose the vast majority of sedimentary rocks.eight igneous rock forming mineralsQuartzOrthoclaseCa plagioclase
Process Continued TRANSPORTATION AND SORTING: The second process is sorting during transportation. The sand and clay, beginning as a poorly sorted mixture, are separated more and more as they travel down stream away from the source. Quartz sand, which rolls and bounces along the bottom, does not transport as easily as clay which travels in suspension. And the CaCO3 is dissolved and therefore just travels with the water. The result is, during transportation these three weathering products do not transport equally well, and become separated. The final separation takes place at the ocean shoreline (image to right). Here we see river transported sediment entering the ocean. Waves crashing on the beach keep the sediment continuously stirred up. The quartz, being relatively heavy, settles quickly to the bottom, the clay remains in suspension until it drifts to the quieter near shelf, where if finally settles to the bottom to form shale. (see picture next slide) Finally, the dissolved CaCO3 precipitates out of suspension in the far shelf, beyond the range of sand and clay to form limestone. The calcite is deposited because plants and animals extract it from sea water and use it to build their skeletons. After death their calcite skeletons form the limestone sediment.
Final Separation Sorting Where different Materials are separated Finally, the dissolved CaCO3 precipitates out of suspension in the far shelf, beyond the range of sand and clay to form limestone. The calcite is deposited because plants and animals extract it from sea water and use it to build their skeletons. After death their calcite skeletons form the limestone sediment.
How to Classify Sedimentary Rocks A Basic Sedimentary Rock Classification A good classification is based on some theory that explains how the rocks form, and are related to each other. We want to group together rocks that form by similar processes. The most general theoretical model we have for sedimentary rocks is the simple ideal model. The basic classification is based on that model. The complication with sedimentary rocks is they form from such a diversity of processes that straight forward classifications are difficult. They are unlike igneous rocks where a relatively straight forward Texture and Composition classification leading directly to interpretation is possible. theorysimple ideal modelTexture and Composition
Reasoning Behind Classification The simple ideal model for the evolution of sedimentary rocks says there are three end products, three attractors, that all sedimentary processes are working to reach - quartz sandstone, shale, and limestone. The three attractors in the simple ideal model are not isolated, however; each one stands for a class of weathering products. For example: Quartz sandstone = all visible grains, including such ones as incompletely weathered feldspar from the granodiorite in the simple ideal model. Shale = all clay sized grains (clay is a generic name; there are many kinds of clay minerals as well as other minerals that are clay sized) Limestone = all dissolved minerals, including not only calcite CaCO3, but also halite (table salt; NaCl), and gypsum (CaSO4. H2O) among others. On the chart below we can see these additional weathering productsgranodiorite
Classification of Sedimentary Rocks So far, so good. But now we have to do some mixing and splitting. Sedimentary rocks are generally divided into three great categories, siliciclastic (or simply, clastic) rocks, chemical rocks and biochemical rocks. Their relationships to the three divisions from the simple ideal model are shown in the figure below. Observe how visible grains and clay sized grains mix together to form clastic rocks, while minerals in solution split to form chemical and biochemical rocks.
Classification of Sedimentary Rocks
Silicicastic Sedimentary Rocks (Silici)Clastic rocks are composed of weathering products that do not dissolved into water, have silica (SiO2) as one of their major components, and are transported either by rolling along the bottom, or in suspension. Because of this the VISIBLE GRAIN and CLAY SIZED GRAIN weathering products in the chart above tend to be mixed, and deposited together. So we group them together as SILICICLASTIC rocks
Silicicastic Sedimentary Rocks Continued Clastic rocks (sandstones, shales, etc.) are classified on two criteria - texture (grain size), and composition (that is, QFL). Shale, of course, is both fine grained, and composed of clay, so the name "shale" incorporates both properties of texture and composition. For the coarser sizes, where the mineral grains can be seen by eye or with a handlens, a complete name consists of two parts. For example, an " arkose sandstone " is a rock composed of sand sized particles, and a large percentage of those particles are composed of feldspar..QFL
WENTWORTH SCALE. Clastic particles are divided into size categories based on the WENTWORTH SCALE. This scale has been in use for over a hundred years and is universally recognized. The middle size is 2 mm, the boundary between sand and gravel. Note that the size categories get geometrically larger, and smaller, from 2 mm. Sand, for example, ranges from 1/16 mm to 2 mm, while granules range from 2 mm to 4 mm, and pebbles from 4 mm to 64 mm. The reason for this is we want to group together particle sizes that can moved more or less together by running water. The Wentworth scale is straight forward, and with a ruler for scale it is relatively easy to classify the rock. One simplification, and one complication, though. The simplification is, we generally group all the greater than sand sized particles into one category - gravel, unless there is a specific need to distinguish these grain sizes. The complication is, gravel is divided into whether the grains are angular (breccia), or rounded (conglomerate).
Chemical and Biochemical Sedimentary Rocks To deposit minerals that are in solution, they must some how come out of solution and this happens two ways. Either they precipitate directly from sea water (usually because it is evaporating and concentrating the salts) - CHEMICAL ROCKS. Or " plants" and " animals" extract the dissolved minerals from the sea water to make skeletons, their skeletons eventually becoming part of the sediment as BIOCHEMICAL ROCKS. So, minerals in solution split into two categories
CARBONATES: These are composed of the mineral calcite (CaCO3 - calcium carbonate), and are thus all known as carbonates. Note on the chart that there are many of these, that they form by both chemical and biochemical processes, and that they tend to be mixed together in various combinations in the rocks. They are extremely abundant and important.
OTHER CHEMICAL ROCKS : These rocks fall into two categories. Chert is a siliceous rock (composed of SiO2) that forms from the recrystalized skeletons of " animals " (single celled radiolarians, and glass sponges) or single celled " plants " (diatoms, silicoflagellates). And although the silica comes from skeletons to become chert it must be chemically recrystallized, thus putting it in the chemical category (but it is confusing). Rock salt (halite; NaCl) and gypsum (CaSO4. H2O) originally are dissolved in the sea water, thus making the sea salty. When sea water evaporates in a closed area, such as a lagoon, the salt concentration becomes very high, supersaturated, and precipitates out. The process is common in desert areas, with examples today in the Red Sea and Dead Sea in the Middle East, both highly saline.
BIOCHEMICAL ROCKS Peat and coal because they come from plant remains are biochemical rocks, but unlike all the other chemical/biochemical rocks peat and coal always form in the presence of clastic rocks - sandstones and shales. This discussionillustrates part of the difficulty of developing a completely consistent classification. Sedimentary rocks form in so many different ways, from so many different processes that coming up with one scheme that is inclusive and yet straight forward is not easy. There always seem to be exceptions to the rule that have to have to be explained individually
Sedimentary Rocks Features Stratification Internal bedding structures These are sedimentary structures which are best seen looking at a side view of a sedimentary rock or sequence of sedimentary rocks. Stratification (or layering) is the most obvious feature of sedimentary rocks. The layers (or strata) are visible because of differences in the color or texture of adjacent beds. Strata thicker than 1 cm are commonly referred to as beds. Thinner layers are called laminations or laminae. The upper and lower surfaces of these layers are called bedding planes
Graded bedding Graded bedding results when a sediment-laden current (such as a turbidity current) begins to slow down. The grain size within a graded bed ranges from coarser at the bottom to finer at the top. Hence, graded beds may be used as "up indicators".
Cross-beds Cross-stratification is a general term for the internal bedding structure produced in sand by moving wind or water. If the individual inclined layers are thicker than 1 cm, the cross-stratification may be referred to as cross-bedding. Thinner inclined layering is called cross-lamination. Cross-stratification forms beneath ripples and dunes. The layering is inclined at an angle to the horizontal, dipping downward in the downcurrent direction. Hence, cross-beds may be used as paleocurrent indicators, or indicators of ancient current flow directions. Cross-beds usually curve at the bottom edge, becoming tangent to the lower bed surface. The upper edge of individual inclined cross-beds is usually at a steep angle to the overlying bedding plane. Hence, cross-beds may also be used as "up indicators".
Trace fossils Trace fossils or ichnofossils include tracks, trails, burrows, borings, and other marks made in the sediment by organisms. They are bioturbation structures formed as the activities of organisms disrupt the sediment. As organisms tunnel through sediment, they destroy primary sedimentary structures (such as laminations) and produce burrow marks. Bioturbation continuing over a long period of time will thoroughly mix and homogenize the sediment. Through this process, a laminated sediment can be altered to a massive, homogeneous sediment with no readily discernable layering or other sedimentary structures. »Dinosaur tracks