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Alessandro Grippo, Ph.D.

Geology 252, Historical Geology

A key to lab 4, Ancient Sedimentary Environments

(extended) answers to Lab 4 questions Last Updated  •  October 15, 2016    

Question 1, page 32
Graywacke Sandstone: the Bouma sequence is typical of turbidites, which are deposited at the foot of the continental shelf, forming in time what we call the Continental Rise. At this point sand and mud are mixed in the same layer, and this is the characteristic of a graywacke sandstone.

Question 2, page 32
simply put the X at the foot of the continental slope

Figure 4.9, page 35
  1. Alluvial fans
  2. The sediment in the alluvial fans is immature. It is in general not sorted (gravel and sand of all sizes can be found together), not very well rounded (it is common to find angular to-sub-angular to sub-rounded grains) and compositionally immature (presence of many rock fragments with composition very similar to that of the source rock. Quartz is not a dominant component of this sediment.
  3. Other non-marine environments in this image include a playa lake, with visible fine-grained (silt and clay) and evaporitic (mostly halite) sediment

Figure 4.10, page 36
  1. The direction of the tidal current is from (upper) left to (lower) right. You can tell by observing the asymmetric pattern of the ripples: gentle slope to the left vs. steep slope to the right
  2. These ripples are asymmetrical, the two sides have different slope angle.

Figure 4.11, page 36

  1. Micrite
  2. Horizontal, or plane bedding
  3. The plant roots would cause loss of bedding. They would be agents of bioturbation

Figure 4.12, page 37

  1. Marine. Associations of cherts and shale are typically pelagic.
  2. This would be a deep marine environment. In shallow waters there would be plenty of clastic, or terrigenous, material from continents, or presence of shallow-water limestones.
  3. The interbeds of shale suggest interruption of siliceous ooze deposition. This can be caused by variations in the productivity/dissolution/dilution rates in the ocean, which can in turn be originated by plate tectonics, climate change, sea level change, surface and deep ocean current circulation, etc.

Figure 4.13, page 37

  1. Trough cross-bedding (compare to figure 4.4 B, page 32)
  2. Stream channel (check the figure quoted above and text on page 30)

Figure 4.14, page 38

  1. Tabular-planar cross-bedding (compare to figure 4.4 A, page 32)
  2. This sandstone was deposited in a nonmarine (terrestrial) environment, by aeolian processes (again, check the figure quoted above and text on page 30)

Figure 4.15, page 38
  1. Symmetrical ripples
  2. A relatively shallow water basin where currents were moving back and forth (waves at the beach)

Figure 4.16, page 38

  1. Horizontal bedding
  2. This sandstone was deposited in a nonmarine environment because it is a lens-shaped (fluvial) channel fill

Figure 4.17, page 39
  1. Horizontal, or plane bedding
  2. In a marine environment, because dolomite forms normally as alteration of calcite which, in layers like these, can only form in a marine environment

Figure 4.18, page 39
(This exercise was skipped)
  1. Cross-bedding (barely visible; notice faint bedding, at about 45°, in the upper left part of the picture)
  2. In a fluvial environment, possibly an alluvial fan. Wind cannot pick up and move gravel-size clasts

Figure 4.19, page 40

  1. Likely, breaks in sedimentation
  2. Possibly a massive density flow, or simply continuos sedimentation without breaks

Figure 4.20, page 40

  1. a
  2. Graded bedding
  3. This sandstone was deposited in a marine environment. In particular, it was a deep marine environment, and precisely in the continental rise at the foot of the slope. Graded bedding indicates it is a turbidite, which forms as a consequence of a density flow along the slope of a continent.

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