Sedimentary Flashcards
Uses for the study of sedimentary rocks?
The search for hydrocarbons (oil and gas) is an obvious use for sedimentology, but this is not the only use. The search for water and aquifers is dependent on the sedimentology and specifically the pore space (porosity)
Other uses for sediments/sedimentary rocks
BIF’s: Band Ironstone Formation – source of iron ore
Mining for either deep coal or open cast (growth in India and China)
Search for placer deposits such as diamonds and precious stones
Aggregates – limestone sandstone, cement etc……..
Sediment Yields?
the total amount of sediment exported from a drainage basin in a given period of time (Km-2 a-1)
The sediment is commonly referred to as clastic, because it consists of individual clasts (grains, pebbles, boulders etc).
Sedimentary rocks derived from this sediment are referred to as clastic rocks (in decreasing grain size: conglomerates, gravels, sandstones, siltstones, mudstones, clays). Limestones and evaporites are different
Trends of sediment concentration?
globally the largest sediment supply is associated with the World’s largest rivers draining large actively uplifting Mountain belts. • Higher in Asia due to altitude o Himalayas o Orogenic belt • Lower in Africa = craton
Types of sediment fall?
Mass wasting: downslope movement of soil or rock under gravity (slow – creep)
Landslide: rapid downward and outward movement of soil or rock
Basins? Types?
Bowl shaped depressions where sediment accumulates
Main control is tectonics which form them and allow for subsidence
Rift Basin - These basins form where the continental crust is being stretched. During the early phases of rifting the Earth’s surface subsides because it thins as it is stretched.As the rift grows, slip on the border faults drops blocks of crust down, produc-ing low areas and narrow mountain ridges - East Africa Rift
Intracractonic Basin - These tend to develop in the interiors of continents away from tectonic plate margins and mountain belts. They have generally round or oval shapes, and have long geological histories of relatively slow subsidence. One hypoth-esis on their formation is that they are thought to form over areas of previous rifting. When rifting stops, the once hot nd stretched crust starts to cool, contracts and sinks. This sinking is known as thermal sag.
Passive Margin Basin - These form along the margins of con-tinents that are not tectonic plate boundaries. They are usually underlain by a former rift with oceanic crust. Passive margin basins occur because long after the rifting has ceased, the thermal relaxation and subsidence continues. - Gulf of Mexico
Foreland Basins - Foreland basins are associated with regions of compressional tectonics and form adjacent and parallel to mountain belts. They are formed primarily as a result of the downward flexing of the lithosphere in response to the weight of the adjacent mountain belt, though many geological and geodynamic processes combine to control their subsequent evolution. The foreland basin receives sediment that is eroded off the adjacent mountain belt, filling with thick sedimentary successions that gradually decrease in thickness away from the mountain belt. - Persian Gulf
Strike-slip basins - Pull apart basin in areas of strike slip faults - San Andreas
Basics of sediments?
• Sediment is the collective term for loose fragments of rocks or minerals formed from erosion and or from direct precipitation
• Initially unconsolidated
• Transportation sees weathering and change of character of sediment
• Rounding
• Sorting
Wentworth classification - based on grainsize - conglomerate, sandstone, siltstone, claystone
Phi is the –log2d where d is grain size in mm.
Types of sediments?
Clastic sedimentary rocks
Conglomerates and breccia (>2mm)
Sandstones
Mudrocks, mudstones, shales (>50% silt or clay size)
Biogenic sediments: a sedimentary rock formed of skeletal fragments of calcite or aragonite and commonly known as a limestone. However, the direct precipitation of calcium carbonate from warm supersaturated, shallow highly agitated marine waters can leads to ooids and oolitic limestone.
Organic sedimentary rocks: form from the accumulation and lithification of organic debris, such as leaves, roots, and other plant or animal material. Good examples include coal, oil shales and shale gas. Bioclastic limestone, and skeletal limestone are also technically organic sedimentary rocks but are usually grouped with the other limestones as being biogenically precipitated.
Chemogenic sediments: Chemical sedimentary rock forms when mineral constituents in solution become supersaturated and inorganically precipitate. Common chemical sedimentary rocks include evaporites such as gypsum, halite, baryte, sylvite
Volcanicastic sediments
Facies and Walthers Law
individual layers or strata change their charac-teristics laterally: they are not uniform along their entire extent and undergo a facies change. A faciesis a distinctive packet of sedimentary rock with specific sedimentary features that distinguish it from other facies
Walther’s Law states that the vertical succession of facies reflects lateral changes in environment. Conversely, it states that when a depositional environ-ment ‘migrates’ laterally, sediments of one depositional environment come to lie on top of another
Transgressions (sea level rise) fines upward
Regressions (sea level fall) fines downward
Flow hydraulics?
Grain density and size Grain drag Grain roughness Flow velocity Flow viscosity Slope Pebble Clusters • Shows flow direction • Larger pebbles on opposite flow direction side
Sedimentary structure basics?
Below 1cm = laminae
Above = Bed
Physical processes such as currents are responsible for most sedimentary structures, but chemical reactions and biological activity also contribute
Sedimentary structures: Marks, traces and sediment disturbances preserved in sedimentary strata
Types of sedimentary structures: Depositional structures – bedforms Erosional structures Biogenic structures Chemogenic structure Internal structures Post depositional features
Way up?
The technique is particularly important in compressional orogenic belts where sediments can be easily overturned.
Geopetal?
Filled in cavities (such as shells) act as sprit levels (Paleohorizontal)
1. Original calcium carbonate shell of living animal
2. Mud that infilled shell after animal had died
3. Crystals that grew in empty space as shell was buried
4. Contact between mud and crystals show the orientation of the ancient seafloor
Pore spaces on top side of grains
Bedform structures?
features formed by the interaction of either water or wind and sediment on a bed surface
A bedform is a feature that develops at the interface of fluid and a moveable sediment dominated bed, the result of bed material being moved by fluid flow. Examples include ripples and dunes on the bed of a river or sea floor.
Flute marks - bulbous upstream end that flares downstream - tapered end pointing down-current and the steep end up-current
Current ripples – unidirectional flow
• Steep slopes before eddies show flow direction - steep slope is direction of flow
Wave ripples – bidirectional
• Tapering base
• Truncated top
Cross bedding
• Planar and trough cross bedding are two of the most commonly recognised types of cross-stratification
Biogenic structures?
The action of plants and animals provides many structures
Trace Fossils (ichnofacies): sediment living organisms
Internal structures?
Graded Bedding: grain size distribution in a regular fashion
Common – normal grading - decrease in energy during deposition – bigger at bottom – thins up
Less common – inverse grading - occurs in debris flows and aeolian ripples – bigger at top, thins down
