A2 Sedimentary Processes Flashcards
Turbidity current
high-velocity current that flows down gentle gradients because the sediment dispersed within it makes it denser than sea water. These currents are triggered by earthquakes or slope instability.
Turbidites
an upward fining deposit of greywacke deposited from a turbidity current.
Bouma sequence
an idealised sequence of sediments and sedimentary structures seen in a turbidite deposit.
Climbing ripples
a series of cross-laminae formed by superimposing migrating ripples. They form when deposition exceeds the rate of migration of ripples.
Tool marks
impressions made on the surface of soft sediment by the dragging or bouncing of an object (tool) in a current.
Rip-up clasts
pieces of shale or mudstone eroded by a current containing suspended sediment. They are preserved when the current deposits its sediment.
Siliceous ooze
pelagic clay containing >30% biogenic skeletal material made of silica.
Plankton
minute organisms living in the surface layers of the ocean, that are transported by currents.
Diatoms
planktonic algae that secrete siliceous material.
Distributary
a stream channel that takes water away from the main stream channel
Cyclothems
represent layers repeated due to cyclic sedimentation.
Topsets
the uppermost horizontal layers of a delta commonly consisting of channel sandstones, coal and seat earth.
Seat earth
a sandy or clay-rich fossil soil found beneath a coal seam. It represents the soil in which coal-forming plants grew and frequently contains carbonised traces of plant roots.
Foresets
the inclined layers formed on the delta front, commonly consisting of cross- bedded sandstones.
Mouth bars
crescent-shaped deposits of sand and silt forming below sea level, where distributaries enter the sea.
Bottomsets
the lowest horizontal layers of a delta, commonly consisting of shales.
Bioturbation
the disturbance of sediment by the activities of organisms (e.g. burrowing).
Banded iron formations
units of sedimentary rock of Precambrian age consisting of alternating layers of iron oxides and chert.
Archean
an era within the Precambrian, covering a period from 4.0 billion to 2.5 billion years ago.
Palaeoproterozoic
an era within the Precambrian, covering the period from approximately 2.5 billion to 1.6 billion years ago.
Great Oxygenation Event (GOE)
the biologically induced appearance of dioxygen (02) in Earth’s atmosphere. It occurred around 2.3 billion years ago.
Photoferrotrophs
photosynthetic bacteria that oxidise ferrous iron Fe2+ into ferric iron Fe3+ using sunlight to create iron oxides and hydroxides instead of molecular oxygen.
Pycnocline
the boundary separating two liquid layers of different densities in the oceans. It forms a lower limit to turbulence caused by mixing processes at the surface.
