Sedimentary Rocks— The Archives of Earth History

Chapter 6 Sedimentary Rocks—The Archives of Earth History

History from Sedimentary Rocks • How do we know whether sedimentary rocks were deposited on • continents—river floodplains or desert sand dunes? • at the water's edge? • in the sea? • Sedimentary rocks • preserve evidence of surface depositional processes • and many contain fossils • These things give clues to the depositional environment • Depositional environments are specific areas • or environments where sediment is deposited

Arches National Park, Utah • Delicate Arch • is one of several arches, pillars, and spires in • Arches National Park • in the Jurassic-aged Entrada Sandstone • which were formed by weathering and erosion • along large fractures called joints

Sedimentary Rocks • Sedimentary rocks • preserve evidence • of the physical, chemical and biological processes • that formed them • Some sedimentary rocks are resources, or contain resources • petroleum • natural gas • coal • phosphorus

Phosphorous • For instance, phosphorous • from phosphorous-rich sedimentary rocks • is used in • fertilizers • animal-feed supplements • metallurgy • matches • ceramics • preserved foods

Investigating Sedimentary Rocks • Observation and data gathering • by visiting rock exposures (outcrops) • and carefully examining • textures • composition • fossils (if present) • thickness • relationships to other rocks • Preliminary interpretations in the field may be made • For example: • red rocks may have been deposited on land • whereas greenish rocks are more typical of marine deposits • (Caution: Exceptions are numerous!)

Investigating Sedimentary Rocks • More careful study of the rocks is needed: • microscopic examination • chemical analyses • fossil identification • interpretation of vertical and lateral facies relationships • comparison with present-day sediments • When data have been analyzed, geologists make an environmental interpretation

Composition of Detrital Rocks • Very common minerals in detrital rocks: • quartz, feldspars, and clay minerals • Detrital rock composition tells • about source rocks, • not transport and deposition • Quartz sand may have been deposited • in a river system • on a beach or • in sand dunes

Composition of Chemical Sedimentary Rocks • Composition of chemical sedimentary rocks • is more useful in revealing environmental information • Limestone is deposited in warm, shallow seas • although a small amount also originates in lakes • Evaporites such as rock salt and rock gypsum • indicate arid environments • where evaporation rates were high

Grain Size • Detrital grain size gives some indication • of the energy conditions • during transport and deposition • High-energy processes • such as swift-flowing streams and waves • are needed to transport gravel • Conglomerate must have been deposited • in areas where these processes prevail • Sand transport also requires high-energy transport • Silt and clay are transported • by weak currents and accumulate • only under low-energy conditions • as in lakes and lagoons

Sorting and Rounding • Texture refers to the size, distribution, shape, and arrangement of clasts • Sorting and rounding are two textural features • of detrital sedimentary rocks • that aid in determining depositional processes • Rounding is the degree to which • detrital particles have their sharp corners and edges • warn away by abrasion • Gravel in transport is rounded very quickly • as the particles collide with one another • Smaller particles in suspension • are usually not as rounded

Rounding • All of these stones are rounded • and have lost their sharp edges • The stone in the upper left is also spherical

Sorting • Sorting refers to the variation • in size of particles making up sediment or sedimentary rocks • If the size range is not very great, • the sediment or rock is well sorted • If they have a wide range of sizes, • they are poorly sorted • Wind has a limited ability to transport sediment • But glaciers can carry any size particles • Glacial deposits are poorly sorted, wind deposits are well-sorted

Rounding and Sorting • A deposit • of well rounded • moderately sorted gravel • Versus a deposit • of angular • poorly sorted gravel

Sedimentary Structures • Sedimentary structures are • features that formed at the time of deposition or shortly thereafter • and are manifestations of the physical and biological processes • that operated in depositional environments • Structures • seen in present-day environments • or produced in experiments • help provide information • about depositional environments of rocks • with similar structures

Bedding • Sedimentary rocks generally have bedding or stratification • Individual layers less than 1 cm thick are laminations • common in mudrocks • Beds are thicker than 1 cm • common in rocks with coarser grains

Graded Bedding • Graded bedding is common in turbidity current deposits • which form when sediment-water mixtures flow along the seafloor • Some beds show an upward gradual decrease in grain size, known as graded bedding • As they slow, • the largest particles settle out, • then smaller ones

Cross-Bedding • Cross-bedding forms when layers come to rest • at an angle to the surface • upon which they accumulate • as on the downwind side of a sand dune • Cross-beds result from transport • by either water or wind • The beds are inclined or dip downward • in the direction of the prevailing current • They indicate direction of the flow of ancient currents

Cross-Bedding • Individual beds are deposited at an angle • Horizontal bedding and cross-bedding in Upper Cambrian St. Peter Sandstone in Wisconsin

Ripple Marks • Small-scale alternating ridges and troughs • known as ripple marks are common • on bedding planes, especially in sandstone • Current ripple marks • formed from wind or water flow • and have asymmetry • indicating the original flow direction • Wave-formed ripple marks • result from the to-and-fro motion of waves • and tend to be symmetrical

Current Ripple Marks • Ripples with an asymmetrical shape • The photo shows current ripples • that formed in a small stream channel • with flow from right to left

Wave-Formed Ripples • As the waves wash back and forth, • symmetrical ripples form • The photo shows wave-formed ripple marks • in shallow seawater

Mud Cracks • When clay-rich sediments dry, they shrink • and crack into polygonal patterns • bounded by fractures called mud cracks • Mud cracks require wetting and drying to form, • as along a lakeshore • or a river flood plain • or where mud is exposed at low tide along a seashore

Ancient Mud Cracks • Mud cracks in ancient rocks • in Glacier National Park, Montana • Mud cracks can fill in • with sediment • when they are preserved • as seen here

Biogenic Sedimentary Structures • Biogenic sedimentary structures include • tracks • burrows • trails • called trace fossils • Extensive burrowing by organisms • is called bioturbation • and may alter sediments so thoroughly • that other structures are disrupted or destroyed

Bioturbation • U-shaped burrows • Vertical burrows

Bioturbation • Vertical, dark-colored areas in this rock are sediment-filled burrows

No Single Structure Is Unique • Sedimentary structures are important • for environmental analyses • but no single structure is unique to a specific environment • Example: • Current ripples are found • in stream channels • in tidal channels • Environmental determinations • are usually successful with • associations of groups of sedimentary structures • taken along with other sedimentary rock properties

Sedimentary Structures • Sedimentary structures and fossils • allow geologists to resolve the history of an area • when rocks have been deformed • Here, the mudcrack “V” opens toward younger strata, • and shape of current ripple marks • Indicate that the youngest layer is lower right

Geometry of Sedimentary Rocks • The three-dimensional shape or geometry • of a sedimentary rock body • may be helpful in environmental analyses • but it must be used with caution • because the same geometry may be found • in more than one environment. • Geometry can be modified by sediment compaction • during lithification • and by erosion and deformation • Nevertheless, it is useful in conjunction • with other features

Blanket or Sheet Geometry • Some of the most extensive sedimentary rocks • in the geologic record result from • marine transgressions and regressions • The rocks commonly cover • hundreds or thousands of square kilometers • but are perhaps only • a few tens to hundreds of meters thick • Their thickness is small compared • to their length and width • Thus, they are said to have • blanket or sheet geometry

Elongate or Shoestring Geometry • Some sand deposits have an elongate or shoestring geometry • especially those deposited in • stream channels • or barrier islands

Other Geometries • Delta deposits tend to be lens shaped • when viewed in cross profile or long profile • but lobate when observed from above • Buried reefs are irregular • but many are long and narrow • or rather circular

Fossils—The Biologic Content of Sedimentary Rocks • Fossils • are the remains or traces of prehistoric organisms • can be used to establish biostratigraphic units • are important constituents of rocks, sometimes making up the entire rock • are important for determining depositional environments • Some rocks, especially limestones, are composed • largely of shells of marine-dwelling animals • or even the droppings of these organisms

Fossils Are Constituents of Sedimentary Rocks • This variety of limestone, • known as coquina, • is made entirely of shell fragments

Fossils in Environmental Analyses • Did the organisms in question live where they were buried? • Or where their remains or fossils transported there? • Example: • Fossil dinosaurs usually indicate deposition • in a land environment such as a river floodplain • But if their bones are found in rocks with • clams, corals and sea lilies, • we assume a carcass was washed out to sea

Environmental Analyses • What kind of habitat did the organisms originally occupy? • Studies of a fossil’s structure • and its living relatives, if any, • help environmental analysis • For example: clams with heavy, thick shells • typically live in shallow turbulent water • whereas those with thin shells • are found in low-energy environments • Most corals live in warm, clear, • shallow marine environments where • symbiotic bacteria can carry out photosynthesis

Microfossils • Microfossils are particularly useful • because many individuals can be recovered • from small rock samples • In oil-drilling operations, small rock chips • called well cuttings are brought to the surface • These cuttings rarely • contain complete fossils of large organisms, • but they might have thousands of microfossils • that aid in relative dating and environmental analyses

Trace Fossils In Place • Trace fossils, too, may be characteristic of particular environments • Trace fossils, of course, are not transported from their original place of origin

Depositional Environments • A depositional environment • is anywhere sediment accumulates • especially a particular area • where a distinctive kind of deposit originates • from physical, chemical, and biological processes • Three broad areas of deposition include • continental • transitional • marine • each of which has several specific environments

Depositional Environments Transitional environments Continental environments Marine environments

Continental Environments • Deposition on continents (on land) might take place in • fluvial systems – rivers and streams • deserts • areas covered by and adjacent to glaciers • Deposits in each of these environments • possess combinations of features • that allow us to differentiate among them

Fluvial • Fluvial refers to river and stream activity • and to their deposits • Fluvial deposits accumulate in either of two types of systems • Braided stream system • with multiple broad, shallow channels • in which mostly sheets of gravel • and cross-bedded sand are deposited • mud is nearly absent

Braided Stream • The deposits of braided streams are mostly • gravel and cross-bedded sand bodies • Mud is nearly absent

Braided Stream Deposits • Braided stream deposits consist of • gravel • cross-bedded sand • but mud is rare or absent

Fluvial Systems • The other type of system is a meandering stream • with winding channels • mostly fine-grained sediments on floodplains • cross-bedded sand bodies with shoestring geometries • point-bar deposits consisting of a sand body • overlying an erosion surface • that developed on the convex side of a meander loop

Meandering Stream • Meandering stream deposits • are mostly fine-grained floodplain • sediments with subordinate sand bodies

Meandering Stream Deposits • In meandering stream deposits, • fine-grained floodplain sediment is common • with subordinate sand bodies

Desert Environments • Desert environments contain an association of features found in • sand dune deposits, • alluvial fan deposits, • and playa lake deposits • Windblown dunes are typically composed • of well-sorted, well-rounded sand • with cross-beds meters to tens of meters high • land-dwelling plants and animals make up any fossils

Alluvial Fans and Playa Lakes • Alluvial fans form best along the margins of desert basins • where streams and debris flows • discharge from mountains onto a valley floor • They form a triangular (fan-shaped) deposit • of sand and gravel • The more central part of a desert basin • might be the site of a temporary lake, a playa lake, • in which laminated mud and evaporites accumulate

Sedimentary Rocks— The Archives of Earth History