A clastic sedimentary rock, sandstone, with generally
coarse and angular grains of quartz and feldspar set in a
compact clay-rich matrix. It may have clasts of many other
minerals and rock fragments and typically has a dark or light
gray color. Graywacke often occurs as laterally continuous
beds interbedded with shale or slate in thick flysch sequences,
and many graywacke beds exhibit sedimentary structures
known as Bouma sequences, indicating deposition by submarine
turbidity currents. Sedimentary structures in Bouma
sequences typically include a graded base to a bed, which may
show scour marks or other markings such as sole marks or
load casts on the bottom of the bed. This lower part of the bed
grades up through an interval of parallel laminations, and then
an interval with ripple cross-laminations. In a complete Bouma
sequence, these are followed upward by an upper layer with
parallel laminations, and then a pelitic top to the bed.
The immaturity of graywacke sediments is generally
interpreted to reflect that they were eroded, transported, and
buried so quickly that chemical weathering did not have a
chance to break down the feldspars and separate the clays
from the quartz grains. These conditions are met in an actively
deforming orogenic belt, and graywackes and flysch belts
are typically associated with the erosion of active or recently
formed orogenic belts. The sediments are typically eroded in
the mountains, transported by streams, and deposited in submarine
foreland basins in front of the deforming orogen.
See also FLYSCH; SANDSTONE; SEDIMENTARY ROCKS.
Tidak ada komentar:
Posting Komentar
Catatan: Hanya anggota dari blog ini yang dapat mengirim komentar.