Interpreting The Sedimentary Rocks Page 2
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1
2Step 2: Look for these physical features to tr
y to fi
gure out the depositional environment.
A.
Rock Color
Oxygen-poor, stagnant settings and high org
anic
Black
Enriched in organic matter & pyrite (FeS
)
productivity
2
Coal swamp or restricted basin
Oxidizing conditions, often asso
ciated with
Red
Enriched in iron oxid
es such as hematite (Fe
O
)
subaerial exposure & hot arid climates.
2
3
Terrestrial or shallow marine
B. Surface markings
Environment was affected by flowing water, wave
Ripple
Small-scale ridge of sand
action, or wind motion
e.g., river, estuary, sand dune
Polygonal-shaped cracks form
ed in mud that has
Environment affected by altern
ating wet and dry
Mudcrac
k
dried out in a terrestrial envir
onment
conditions
Tidal flat, lake shore, desert
C. In
ternal bedding features
Settling of sediment in suspension
Lake: Seasonal deposition of fine (winter) and
Very fine layering composed of discrete layers
Lamination
coarse (summer) sediment
of sediment a millimeter or so in thickness
Marine: Indicates lack of bioturbation and
therefore stressed conditions, e.g., low o
xygen.
Fining-upward - deposition from a waning current
(turbidity flow)
Graded
Upward gradation in grain size from coarser
Coarsening-upward – deposition under
bedding
or finer material
increasingly high energy conditions (environment
is shallowing)
Environment was affected by flowing water, wave
Cross-
Formed by the migration of the slip-faces of
action, or wind motion
bedding
ripped bedforms or dunes
e.g., river, estuary, sand dune (large-scale cro
ss-
beds)
Deposition in environment where current flow
Heterolithic
Closely interbedded deposit of sand and mud
varies considerably
bedding
Tidal flat, estuary
Convolute
Folds whose intensity dies out both upwards
Subjection of water-rich sediments to an exte
rnal
bedding
and downwards within a single bed
shock (earthquake, large waves)
Step 3: Check
yourself b
y rea
ding
about the depositional en
viron
ment
you inferred.
Comm
on
Enviro
nment
Sedimentary Structures
Fossils
Lithologies
Terrestrial
Sandstone
Unidirectional ripples and/or
Rare
River
cross-bedding; channel forms in
cross-section
Terrestrial
Mudrock, shale,
None or lamination; evidence for
Plant rootlets; coaly seams
,
Floodplain
siltstone
soil development
plant debris
Terrestrial
Mudrock, shale,
None or lamination
Rare;
microscopic fossils
Lake
siltstone, limestone
Bioturbation on bed surfaces
such as diatoms
Terrestrial
Ripples; large-scale cross-
Sandstone
Rare
Sand dune
bedding
Ripples and/or cross-bedding;
Often rare; o
rganisms that
Coastal
channel forms
in cross-section
can withstand brackish
Sandstone
Estuary
Evidence for tidal influenc
e;
conditions
Bioturbation
Mudrock, shale,
Common; organism
s that
Offshore/shelf siltstone; sandstone
Common bioturbation
require normal marine
interbeds possible
conditions to live
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