Page 190 - Laboratory Manual in Physical Geology

SEDIMENTARY STRUCTURES

ILLUSTRATIONS DESCRIPTIONS ENVIRONMENTS

Raindrop RAINDROP
impressions IMPRESSIONS: Raindrop impressions
Tiny craters formed by occur on muddy land
raindrops as they impact surfaces.
bedding plane surfaces.

Horizontal strata occur
Horizontal HORIZONTAL where sediments settle
strata
STRATA: from a standing body of
Relatively flat beds water or air; or where
(≥ 1cm thick) and currents travel parallel to the
laminations (< 1cm thick). surface on which sediments
are accumulating.

Graded beds form when
GRADED BED: a turbulent body of
Stratum that contains water full of sediment
different sizes of sedimentary (flood, wave, river)
grains arranged from largest suddenly loses energy
Graded beds
at the bottom of the bed to and calms down. Large
smallest at the top. particles settle out
before small.
Flow direction
Current ripple
marks (air or water)
CURRENT Current ripple marks form
RIPPLE MARKS: in any environment where
Asymmetrical ripple marks. wind or water travels in
The steep slope faces down one direction for some of the
current, and the gentle slope time: rivers, ocean currents,
faces up current. wind blowing sand dunes.

Cross-bedding

CROSS-BEDDING: Cross-bedding forms
Inclined beds or wherever there are wind
laminations. or water currents.
inclined
to
right
Bimodal
cross-
bedding
inclined
to BIMODAL Bimodal cross bedding
left CROSS-BEDDING: forms in environments
Sequence of cross-bedding where currents of wind
or water flow back and
in which cross-bedding of forth in opposite
current ripple marks is
Oscillation back and forth (water) directions. It is common
Wave inclined in opposite in environments with tides.
ripple directions.
marks
Wave (symmetrical) ripple
marks form in any body of
WAVE RIPPLE MARKS: water where gentle waves
Symmetrical ripple marks. barely touch bottom, or
where weak currents move
Cross-bedding back and forth (oscillate) in
shallow water.

FIGURE 6.12 Sedimentary structures.

Sedimentary Processes, Rocks, and Environments ■ 169