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Alessandro Grippo, Ph.D.

Geology 252 - Historical Geology

A key to lab 5, Major Tectonic Settings of Earth History

(extended) answers to Lab 5 questions Last Updated  •  November 1, 2016    

question 1, page 44
The question already states that this sediment is a sandstone. It would be an immature, clastic sedimentary rock, classified as a graywacke sandstone: the trench is at the bottom of the ocean, its landward side receives sediment from land, and specifically through turbidity currents. Some pelagic clays could also settle indipendently, from suspension.
Turbidity currents move sediment from the shelf (which is very narrow in the case of an active margin such as the one depicted in Model 1) to the foot of the slope, carving submarine canyons during its descent. The sediment would then come to rest creating deposits called submarine fans. Submarine fans would eventually originate what we call the continental rise. The individual layers, called turbidites, would be characterized by graded bedding.
Along an active margin such as that of Figure 5.1, Model 1, the sediment would accumulate in the space between the slope and the trench, or in the trench itself if it is close enough and there are huge amounts of sediment coming from the shelf (see the case of the trench offshore Oregon and Washington states). It would partially clog it, possibly slowing down subduction.
Along a passive margin such as that of Figure 5.1, Model 2 instead, turbiditic materials would accumulate at the base of the slope, creating the ideal conditions for the formation of the continental rise.

question 2, page 44
The rocks that form from sediment deposited in the subduction trench are not many:

  • the finest components of a turbidite (for instance, the finest sands, the silts and the clays of the Tc-e intervals of the Bouma sequence.
  • pelagic clays can settle here, and they would make a mature sediment.
Rocks that do not form in the trench but might get into the trench (because they are carried passively on top of the pillow basalts formed at a mid-ocean ridge by a moving oceanic plate) are, if and when present, the pelagic limestones, cherts, and the red clays of the deep ocean.
All these rocks would be different from the ones of question 1.

question 3, page 44
Farthest from the source area you would see shales (clastic sedimentary rocks). Unrelated to the source area, depending on other factors, chemical sedimentary rocks such as pelagic cherts and, above all, limestones may form.

question 4, page 44
It would be a very mature, clastic sedimentary rock, classified as a quartz sandstone: model 2 is a passive continental margin, and the sand that is found along the coast and on the shelf tends to be very well rounded and sorted.
Sand-size sediment can be moved on the shelf only if there are strong enough marine currents (see the three models of Fig. 5.4, page 45, on the 9th edition of your lab manual). Exceptional events, such as storms, tsunamis, very powerful tides, etc., can have the same effect (but will cause the formation of different sedimentary structures).

question 5, page 44
Among the factors that may be important in order to determine the relative abundance of sand vs. carbonate on the shelf of a passive margin are the amount of terrigenous influx (clastic sediment) reaching the ocean, and temperature and salinity of the water (which would influence the solubility of CO2 in ocean water and the conditions of life for marine organisms which might use CaCO3 to build a shell, a reef, or another biochemical structure). See point 2, page 40 of your lab manual, and your notes taken during several discussions on this topic in class (e.g. Bahamas/Florida platform, Amazon River mouth, etc.)

question 6, page 44
A turbiditic sandstone (graywacke sandstone), or turbidite. The bulk of a facies association called a flysch.

question 7, page 44
In Model 1 (active margin), where the shelf is very narrow, most of the sediment would probably move down the slope and deposit by the trench or into it. In Model 2 (passive margin), where the shelf is much wider and the sediment source area is much more distant than in the previous model, most of the sediment would likely settle on the shelf itself.

question 8, page 44
Possibly, a continuous, steady subsidence. An example could be provided by the carbonate platform of the Bahamas, that has developed on the passive margin of North America, in the Atlantic Ocean. This platform started to develop at the time of the opening of the Atlantic Ocean, about 200 million years ago, and keeps on developing today. The oldest carbonates are found at the bottom of the ocean at pelagic depth. Those carbonate formed in shallow waters, indicating that 200 m.y. ago those were shallow waters and that continuous, steady subsidence has been going on during all this time.

question 1, page 47
1a - quartz, clastic, very mature sandstone; horizontal bedding, minor amounts of cross-bedding
1b - there is no shale because shale moves as suspended load and it is taken offshore by waves and coastal currents

question 2, page 47
2a - fragmented fossils occur because currents were strong enough to break the shells: in this specific case, shallow marine currents (documented by the presence of submersed dunes, and arrows saying "marine currents"). These currents can interfere with the bottom only in relatively shallow waters. Whole fossils occur away from the coast, where waters get deeper and currents are either absent or too weak to break the shells. Based on this, you can confidently assume that the open ocean is to the left and the landward side to the right.
2b - cross-bedding (cross-bedding is a sedimentary structure that, in this context, forms in sands because of a shallow water current). The composition of sand (carbonate, rather than quartz as seen before) does not influence the type of sedimentary structure that may form.
2c - The most important differences are related to the water energy (velocity of currents) in the different environments.

question 3, page 47
3a - scant precipitation, high evaporation rates: a dry climate
3b - the most common evaporitic minerals are halite and gypsum, but several others might occur
3c - the western part of the basin is in contact with the open ocean. Water enters from the west and evaporates in the basin, but tends to be replaced unevenly within the basin itself. Limestone is a carbonate that can also form inorganically, and hence it can crystallize without requiring the complete evaporation of water from the basin.

question 1, page 48
1a - the source area is a coastal mountain chain constituted by an igneous/metamorphic complex (see figure) and as a consequence, the sandstone did not have time to weather completely: it will be a clastic immature sediment, containg possibly rock fragments and minerals such as quartz, clay, K-feldspar and mica flakes. Graded bedding is characteristic of turbidites.
1b - A turbidity current would likely erode previously deposited sediments, creating uneven surfaces. More distant deposits (offshore) would be "blanketed" by the finest components of the turbidite (that is, turbiditic shale, or pelite).

question 2, page 48
2a - On the deltaic coastal plain you have the transition between a terrestrial environment, where rivers are active, and a coastal environemnt, where deltas and beaches form. Horizontal bedding and cross-bedding (in sands) are encountered, sediment size tends to be coarser. On the ocean floor you will find mostly clay (shales) in its part closest to the continental margin and bedding would only be horizontal.
2b - Quartz and any other mineral found in sand. On the ocean floor you will mostly have clay minerals, which are clay size. Quartz grains, usually the size of sand or silt, are most of the times too coarse to be carried to the ocean bottom.

question 3, page 48
3a - Setting C is an orogenic one, with a basin delimited by active normal faults and surrounded by mountains. As a consequence, erosion rates would be high, and the clastic sediment very immature, rich in rock fragments and minerals originating in the surrounding mountain chains. Sorting would not be high but there will be a clear separation between gravel and coarse sand at the edge of the basin, and finer sediment towards its center, depending on how much water would be available for erosion and transportation. Bedding can be horizontal in the basin but one can assume that cross-bedding can be seen in the alluvial fans at its edges.
3b - The arkosic conglomerate would contain more rock fragments and would possibly be more immature than the arkosic sandstone, which would have traveled more and hence been exposed longer to weathering agents.

figure 5.6, page 49

  1. association 1 (check the description, and confront it with the lithologies described in this figure)
  2. horizontal bedding; hints of cross-bedding (in oolites)
  3. cratonic
  4. shelf (presence of oolites, bryozoans, algae, clastic limestones)
  5. because the bryozoan beds (an the fine-grained limestones) are typical of a relatively low energy environments, like shales; oolites instead generally require high energy.

figure 5.7, page 49

  1. association 2 (presence of coal)
  2. plane, horizontal bedding
  3. cratonic (shelf)
  4. non marine to shallow marine
  5. transgression

figure 5.8, page 50

  1. association 4 (presence of breccia and conglomerate)
  2. orogenic
  3. the breccia would form at the base of a fault scarp, with no or little erosion or transportation; the conglomerate would form when materials are eroded and transported by a stream. This association would probably indicate different stages in the life of a basin: during tectonic activity episodes, breccia is generated, while during tectonic stasis interval, materials are eroded and transported, hence becoming rounded.

The following exercises were not part of the assignment, but here is a key to all questions anyway:

figure 5.9, page 50
From the description (immature sandstone with ripples and plant fragments, and darker, thinly bedded silty shale), this is one of the most difficult images to interpret.

  1. likely association 4
  2. horizontal layers
  3. orogenic (shelf)
  4. likely non-marine

figure 5.10, page 50
The description includes conglomerates and immature sandstones, in association with relatively deep (600 to 1200 m) shale. Except for the conglomerates, everything else points towards relatively deep waters, where a sandstone can only occur as a turbidite. Occasionally, conglomerates can be taken to these depths by turbidity currents. The conglomerate existed in a coastal area before becoming involved with a current of this kind.

  1. likely association 5
  2. shallow marine
  3. orogenic (shelf)
  4. via submarine channels, possibly as part of a turbidity current

figures 5.11A and 5.11B, page 51

  1. association 5
  2. orogenic
  3. erosion caused by the turbidite on the previously deposited bed
  4. continental rise, at the base of the slope, marine

figure 5.12, page 51

  1. association 1
  2. cratonic
  3. possibly, a drop in sea-level (regression)

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