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Download Sedimentary Rocks and Depositional Environments
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Sedimentary Rocks and Depositional Environments The Rock Cycle! YOU SHOULD KNOW THIS FROM YOUR PREVIOUS GEOLOGY CLASS . Igneous Rocks • Cooling and crystallization of magma or lava • Large or small crystals Metamorphic Rocks • Extreme heat and pressure alters existing rock into a new rock • Foliation lines and cleavage planes, or non-foliated Sedimentary Rocks • Rocks formed by deposition of sediment at the Earth’s surface or within bodies of water Why are sedimentary rocks important to historical geology? • Principal repository for information about the Earth’s past – – – – Environment of deposition Fossil Record Tectonic history Climate reconstructions • Why should we care? – Formational processes– chemical, biological, physical – determine the sedimentary rock that forms – Provides knowledge of paleo-environments – Climate clues – past is the key to the future Types of Sedimentary Rocks 1. Inorganic/Chemical • Comprised of minerals, chemically precipitated from an aqueous solution or left behind via evaporation 2. Organic • Comprised mostly of the remains of once-living organisms (plant or animal) 3. Clastic (Made of pieces) • • Comprised mostly of clasts (pieces broken off of a preexisting rock) Weathering + transport by either wind, ice, or water + deposition + compaction by new sediment deposited on top + cementation of clasts by groundwater = NEW ROCK Texture of Clastic Sedimentary Rocks • Size, shape, orientation of clasts or minerals in a rock • Used to determine the velocity, direction of the current in the environment where the rock was formed • Examples: Fine clays can only settle in deep still water Gravel and larger clasts are only deposited by high energy, rapidly moving water Use the rocks to interpret their depositional environment! Size Roundness Sorting Examples of Clastic Depositional Environments Alluvial fans Rivers & Braided stream Lakes Meandering streams Barrier island marsh, dunes, beach Shallow marine – delta, shelf Conglomerate Describe the grains of this clastic rock. Where do you think it formed? Conglomerates form in Alluvial Fans Braided Stream Conglomerates also form in braided streams Conglomerates in Gravel Bars formed by Braided Streams Lakes … are low-energy environments that allow fine-grained sediments to settle … and therefore form fine-grained sedimentary rocks! Shale Meandering Rivers Cut bank = water speeds up around outer curve = erosion Point bar = water slows down around inner curve = deposition Channel sandstone in meandering rivers Marshes/swamps/bogs form very characteristic sedimentary rocks Peat can forms in marshes and swamps • Plant material does not fully decay – anaerobic (lowoxygen) conditions • Amount of decomposition can be used to reconstruct ancient ecology and climate Coal – Swamps or Wetland Forest Peat to Coal: A progression of time and decomposition Peat – Lignite – Bituminous - Anthracite Layers of Coal-bearing sediments Fossiliferous Shale • Can form in marshes and swamps with fine grained sediments (muds and clays), but also in other low energy environments, such as deep lakes or oceans • Determine which by analyzing the fossils present • = more mud than vegetation = a shale with fossils in it, vs coal/peat, which are almost 100% vegetation Offshore = deep water Near Shore = wavedominated shallows Beach shells Shale • Burgess Shale in Canada – important fossil site – soft-bodied preservation of offshore environment over 500 million years ago Coquina Limestone Shell hash from a high energy nearshore/beach environments Fossiliferous Limestone = shells from shallow nearshore environments Reef Formation • Form in shallow, tropical nearshore waters Fossil Coral Reef Fossiliferous limestone = biochemical sedimentary rock) Deep Sea/Offshore A very low energy environment! Chalk Biogenic calcium carbonate sediments – form by accumulation of phytoplankton called coccolithophores = a type of limestone Ancient chalk beds The White Desert of Egypt, Sahara el Beyda, in the Sahara Desert, was once deep underwater and deposits of chalk built up over millennia. Today, this region is above sea level and the wind has eroded the ancient chalk deposits. Evaporite Minerals – Form inorganic chemical sedimentary rocks Halite Gypsum - CaSO4·2H2O Inorganic Chemically Precipitated Sedimentary Rocks Stalagmites and stalactites are inorganically precipitated limestone Evaporite Formation • Precipitates out of hypersaline solution • In evaporating ocean basins such as Caspian Sea, Black Sea, Red Sea and saline lakes such as Great Salt Lake • Indicates a hot dry climate NEXT: Sedimentary Structures Galen Rowell Structures in sedimentary rocks give climate clues • Give climate change clues: What environments were present and when – Glacial processes – unique signatures from glacier advance/retreat – Dry, arid, desert-like conditions – Tropical marine environments Varves Light layer = coarser sediment (silt, fine sand) deposited under higher energy conditions when meltwater introduces sediment load into the lake water. Dark layer = During winter months, when meltwater is reduced, and often when the lake surface freezes, fine clay-size sediment is deposited forming a dark layer. A sedimentary structure preserved in lake or ocean sediments. Each light and dark layer couplet is equal to one year of deposition. Glaciers Glacial Striations Franz Joseph Glacier, New Zealand Approximate distance to glacier: 2 MILES The Wave - on the border of Arizona and Utah, made of 190million-year-old sand dunes that have turned to rock. It is accessible only on foot via a three-mile hike and highly regulated. Crossbeds • sediment deposited at an ANGLE instead of horizontally •form as sediment moves up one side and slides down the lee side = dunes migration • Diagonal beds or inclined surfaces (lines) also show changing wind direction • Erosional surfaces present Sand Dune Cross Beds Fossil Desert Sand Dunes in Utah Crossbeds preserved Crossbedded structures can also be produced by currents in water; these beds are generally less steeply angled/smaller than those found in sand dunes Dry Lake Bed Mud cracks in honeycomb patterns – dried out bed of clay shrinks and leaves cracks, which can be secondarily filled with sediment, then cemented Dry Lake Bed Mud cracks in honeycomb patterns – dried out bed of clay shrinks and leaves cracks, which can be secondarily filled with sediment, then cemented Beach Environment Sedimentary Structures Beach Ripples Ripple mark structures are preserved in Sedimentary Rocks • Ripple marks are preserved in this sandstone from the oscillation of waves on the beach = a shoreline of an ocean or a lake • Note if large or small, asymmetric vs symmetric – tells you if it was formed via wind (eolian) or water current Desert Sand Dunes Ripple Marks can be preserved in a desert environment – asymmetric shape How to proceed thru the lab: • Sketch rock, color, grain size, etc. • IDENTIFY the rock • Look for sedimentary structures – variations in layering or sediment shapes, features in a rock that gives clues to the environment in which deposition occurred – determine/name what you see • Discuss energy and possible depositional environments • Plot lat/long on map • Determine depositional environment – use lab manual for help Quiz next week • Three types of sedimentary rocks and their general characteristics • What rocks are found in what environment (e.g. limestone = deep ocean) • Sedimentary structures in what environment (e.g. crossbeds = dunes or nearshore current) • Rock cycle • Identify rock samples!!
