GEO 101 Lecture Notes - Lecture 11: Ripple Marks, Mudcrack, Cross-Bedding
23 Jun 2018
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Organic Sedimentary Rocks
Organic sedimentary rocks form from the accumulation and lithification of organic
debris, such as leaves, roots, and other plant or animal material. Rocks that were once
swampy sediments or peat beds contain carbon and are black, soft, and fossiliferous.
Rich enough in carbon to burn, coal is an organic sedimentary rock that is a widespread
and important fuel source. Coquina, bioclastic limestone, and skeletal limestone are
also technically organic sedimentary rocks but are usually grouped with the other
limestones as being chemically precipitated.
Sedimentary Features
Features that were part of the sediments when they were deposited are often preserved
when the sediments become lithified. These features are very useful in reconstructing
how the sediment grains were transported, where they came from, the age relationships
of different layers, and what the environment was like when the sediments were
deposited.
Bedding. Bedding is often the most obvious feature of a sedimentary rock and consists
of lines called bedding planes, which mark the boundaries of different layers of
sediment. Most sediments were deposited along a flat surface that was roughly parallel
with the depositional surface. An exception is cross bedding‐, where sediments are
carried over an edge or slope by a strong surge of water or wind, forming steeper
layers. Cross bedding tends to occur locally within a larger block of rock, and is overlain‐
and underlain by flat lying beds.‐ Herringbone cross bedding‐ is a distinctive pattern of
alternating cross bedding directions that is reflective of a rhythmic, high energy ‐ ‐
environment, such as a tidal zone.
Graded beds are common when a sediment is being deposited by a slow moving ‐
current. The base of the bed consists of coarser material, which settles to the bottom
first. The subsequent beds grade upward through sand and silt, to the finest clay sizes
at the top. This pattern is typical in submarine turbidity flows, where sediments are
dislodged and tumble down an ocean floor slope.
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