Chapter 3 Sedimentary Rocks, Sedimentary Environments and Fossils

Chapter 3 – Sedimentary Rocks, Sedimentary Environments and Fossils

3.1 Introduction to Sedimentary Rocks  Sedimentary rocks form at Earth’s surface o Earth’s surface is the interface between the solid Earth, hydrosphere, atmosphere, and biosphere o Therefore, sedimentary rocks are indicative of surface conditions and related climatic feedbacks  There are several common types of sedimentary rocks o Terrigenous – derived from preexisting rocks by weathering, erosion, transport, and deposition of sediment (such as sand or mud) o Biogenic – consist predominantly of shells of dead organisms (fossils) o Chemical – formed by chemical precipitation from water  Besides sediment grain size and composition and fossils, sedimentary rocks may also preserve sedimentary structures as clues to their origin and interpretation 3.2 Processes of Weathering  Rocks exposed at Earth’s surface break down by processes of physical and chemical weathering o Physical weathering involves the formation and enlargement of cracks o Chemical weathering results from weak acids produced by the decay of organic matter and the dissolution of carbon dioxide in water o Ions dissolved from rock are carried by rivers and groundwater to the ocean o In the weathering of granite, feldspar and micas form mud; highly resistant minerals such as quartz are freed to produce sand and gravel 3.3 Terrigenous Sedimentary Rocks  Formation of Terrigenous Sediments o The three broad types of terrigenous sediment are based on the predominant grain size: gravel, sand and mud (not their chemical composition) o Loose rock and mineral grains that result from weathering are transported (by landslides and streams) and eventually deposited, buried and lithified o Lithification involves consolidation of lose sediment into rock by burial, compaction and cementation  What Do Terrigenous Sedimentary Rocks Tell Us About How They Formed? o Sedimentary rocks have histories that can be deciphered using indicators such as the size, shape, and mineral composition of the grains o During transportation and deposition, sediment grain sizes may be deposited together, or may be separated according to size o The separation according to size is the sorting; it depends upon the amount of environmental energy available to transport the grains o Coarse grain sizes (pebbles and cobbles) tend to be deposited close to the source area because they are the least easily transported o Muds (silt and clay) tend to be transported the furthest because they are easily suspended in water; they are deposited in quiet waters

© 2013 Jones and Bartlett Learning, LLC o The grains of sands and gravels may undergo rounding as they are transported due to abrasion of the grains against one another o The extent of transport, rounding and sorting in terrigenous sediment reflects its maturity o Maturity is of two types: compositional (relative content of unstable minerals) and textural (relative sorting and rounding of grains) 3.4 Biogenic Sedimentary Rocks  Biogenic sedimentary rocks are biologic in origin (generated by organisms) o They contain the shells of creatures that use dissolved ions to secrete shells for skeletal support and protection o Limestone, a common biogenic sedimentary rock, is composed of calcium carbonate o Shallow-water limestones form where there is little terrigenous sediment (otherwise it would dilute the calcium carbonate) o Biogenic limestone formed deeper in the ocean from the remains of plankton is called calcareous ooze o Another type of ooze is siliceous ooze, which forms from the siliceous remains of other types of plankton 3.5 Chemical Sedimentary Rocks  Chemical sedimentary rocks are generated by precipitation from water o Evaporites such as gypsum, anhydrite and halite represent a type of chemical sedimentary rock that form when water evaporates o Oolite is a type of limestone formed when calcium carbonate precipitates from water as rounded grains (ooids) o Shallow water limestones may be altered to dolostones by highly saline waters (brines) that percolate through them o Cave formations such as stalactites and stalagmites are made of travertine, and are another type of chemical sedimentary rock 3.6 Marine Environments – environments of the ocean are referred to collectively as marine environments  Marginal Marine Environments – environments where land and sea meet o Associated with coasts, bays, barrier islands, and estuaries o Often are heavily influenced by fresh waters and sediment brought by rivers o Account for a small area of Earth’s surface but are the most productive of all ecosystems o May be stressful to organisms because of fluctuating conditions (for example: salinity, oxygen, and turbidity)  Coral Reefs o Among the most diverse biologic communities on Earth o Thrive in shallow, clear, warm tropical waters of normal marine salinity with little terrigenous influx o Corals secrete massive amounts of calcium carbonate, forming the limestone framework of the reef

© 2013 Jones and Bartlett Learning, LLC o During Earth’s history, other types of organisms besides corals have formed reefs, but also thrived in relatively warm, shallow, clear waters  Continental Shelves, Continental Slopes, and the Abyss o The neritic zone consists of the waters of the continental shelf (from the coast to 200 m depth); it is a gradually sloping region o Beyond the shelf break is the bathyal zone, the waters of the continental slope; its average inclination is actually only a few degrees o Below the lower reaches of the continental slope lie the abyssal plains, which average 4000 m water depth o In certain locations, the abyssal plain may grade into the hadal environment (deep sea trenches), greater than 5000 m depth o All bottom environments of the marine realm are collectively referred to as benthic (and the creatures that live there are the benthos) o The water column is referred to as the pelagic environment; it is occupied by phytoplankton and zooplankton (floating plants and animals) and nekton (swimming species) o Portions of the pelagic zone may be occupied by an oxygen minimum zone; in this zone, decay of dead organic matter results in low levels of dissolved oxygen in the water column o At ocean depths greater than about 4000 m, the level of dissolved carbon dioxide in the water results in the dissolution of calcium carbonate sediments; this is called the CCD (carbonate compensation depth) 3.7 Terrestrial Environments  Forests o Tropical rain forests are among the most productive of Earth’s ecosystems o This was also true in the past, and widespread photosynthesis on land may have increased oxygen levels at times in Earth’s history o Lush vegetation can result in the deposition of peat, which in turn can be turned into coal under burial conditions  Deserts o Deserts develop beneath descending air masses and in the lee of large mountains o Even though there is a lack of rainfall in deserts, water is nevertheless the main agent of erosion, transportation, and deposition  Glaciers o Glaciers occur as alpine glaciers (in mountain valleys) or as much larger ice caps on continents (as on Antarctica and Greenland, where the ice can be several km thick) o There are several types of sediment associated with glaciers, collectively referred to as drift  Lakes o Lakes constitute the lacustrine environment o Lakes may contain annual layers called varves, which are related to seasonal changes in plankton production and sedimentation 3.8 Sedimentary Structures

© 2013 Jones and Bartlett Learning, LLC  Various features – called sedimentary structures – can be used as indicators of the processes and environmental conditions involved in the transportation and deposition of sediments o Examples are mud cracks, ripple marks, cross bedding, and graded bedding 3.9 Fossils  Early Processes of Fossilization o Fossils are the remains or traces of preexisting organisms o Taphonomy is the study of the formation and preservation of fossils o Fossils are normally represented by hard parts such as shells, bones, or teeth, but trace fossils (ichnofossils) such as tracks, trails and burrows also occur  Modes of Preservation o Organisms’ hard parts are subject to several processes after burial; these can grade into one another, and are referred to as petrification o Fossilization process include permineralization, replacement, recrystallization if the hard parts undergo relatively minor changes o More drastic changes to the hard parts may leave a cavity known as a mold; if the cavity fills in, a cast may result o In rare cases, soft parts are preserved by being encased in amber or mummified o Lagerstatten are “mother lodes” of fossils which are not normally preserved; they have been studied intensively to yield information on the history of Earth’s systems