Sedimentary Processes and Environments Questions and Answers

Answer: A

The graph is the Hjulstrom curve.

Filip Hjulstrom's diagram plots flow velocity against grain size and shows the velocity needed to erode, to keep in transport and to deposit particles of each size. It shows that sand of about 0.1-0.5 mm is the easiest to erode, while finer silt and clay need higher velocities because of their cohesion, even though once lifted they settle out only at very low velocities.

A flow duration curve shows how often a given discharge is exceeded, and a sediment rating curve relates sediment load to discharge. 'Trombe's curve' is not a recognised sediment diagram.

Answer: B

The key environmental factor in an estuary is salinity.

An estuary is where fresh river water mixes with sea water, so salinity varies strongly in space (from fresh at the head to marine at the mouth) and in time (with tides, seasons and floods). Most organisms tolerate only a limited range of salinity, so the salinity gradient largely decides which species live where; estuarine faunas are low in diversity and dominated by euryhaline species.

Sediment type, light and temperature also matter, but they vary much less between an estuary and the neighbouring sea or river than salinity does.

Answer: B

The mode described is saltation.

In saltation, grains are lifted a short distance off the bed by fluid lift and drag, carried a little way downstream along a curved, ballistic path, and then fall back to strike the bed, often knocking other grains into motion. It is the typical mode for sand in rivers and in wind.

In suspension the grains are held up by turbulence for long distances and do not follow ballistic hops. Sliding and rolling are traction modes: the grain stays in continuous contact with the bed and is never lifted into the flow.

Answer: B

The feature is a crevasse-splay.

When a flood breaches a natural levee, water pours through the gap (the crevasse) onto the floodplain and spreads out in a fan of small distributary channels that divide and rejoin. The sand and silt it drops form a lobe-shaped crevasse-splay deposit on the floodplain side of the levee.

A debris cone is a steep cone of coarse debris at the foot of a gully. A fan delta is an alluvial fan built directly into standing water. Meander scrolls are the curved ridges left on the inside of a migrating meander bend by successive point bars, not a fan of channels off a levee.

Answer: C

This is herringbone cross bedding.

Where currents reverse direction, as with flood and ebb tides, successive sets of cross-strata have foresets dipping in opposite directions. In section the alternating sets look like the bones of a fish, hence the name, and the structure is a classic indicator of tidal deposition.

Flaser and lenticular bedding are also tidal structures, but they describe mud drapes in ripple-laminated sand (flaser) or isolated sand lenses in mud (lenticular), not opposed foreset directions. Hummocky cross bedding is a storm-wave structure of gently curved, low-angle laminae.

Answer: B

The features describe a Pelagic environment.

Pelagic sediments accumulate slowly in deep ocean basins, far from land and starved of clastic debris. They form thin, laterally continuous sheets of chalk, chert and black shale, built mostly from the shells of planktonic microfossils (calcareous ooze, radiolarian and diatom silica) with clay and wind-blown dust, which matches every feature listed.

Tidal, continental shelf and deltaic environments are shallow and supplied with sand and mud from land, so they produce cross-bedded sandstones, heterolithic beds and coarsening-upward sequences, not homogeneous chalk, chert and shale.

Answer: A

The maximum diameter is about 103.2 µm.

Stokes settling velocity and the grain Reynolds number are

\[ v = \frac{(\rho_{s} - \rho_{f}) g D^{2}}{18 \mu} \]

\[ Re = \frac{\rho_{f} v D}{\mu} \]

Setting \(Re = 1\) and substituting for \(v\):

\[ \begin{aligned} D^{3} &= \frac{18 \mu^{2}}{\rho_{f} (\rho_{s} - \rho_{f}) g} \\ &= \frac{18 \times 0.0001}{1 \times 1.65 \times 981} \\ &= 1.11 \times 10^{-6}\ \text{cm}^{3} \end{aligned} \]

so \(D \approx 0.0103\) cm, about 103-104 µm, which is option A to within rounding.

Larger grains settle fast enough to exceed Re = 1, so 204.1, 322 and 402.1 µm would no longer settle in the laminar (Stokes) regime.

Answer: C

The features describe Alluvial fans.

Alluvial fans build where steep mountain streams emerge onto a basin floor at fault-bounded margins: rift grabens, foreland basins and rising thrust belts. They form wedge-shaped bodies that thin away from the mountain front, with very poorly sorted, immature deposits from boulders to clay, debris-flow beds, fanglomerates and sieve deposits, and they rarely preserve fossils.

Lacustrine deposits are fine-grained and often fossil-rich. Meandering rivers deposit fining-upward point-bar sequences, and braided rivers, though coarse, lack the debris flows, sieve deposits and extreme sorting range listed here.

Answer: B

The shallow stage just after burial is Eodiagensis (eodiagenesis).

Choquette and Pray (1970) divided diagenesis into three stages: eodiagenesis, close to the surface where pore waters are still connected with the depositional environment; mesodiagenesis, during deeper burial; and telodiagenesis, when rocks are uplifted and exposed again to meteoric water.

Mesodiagenesis is therefore the deep stage and telodiagenesis the uplift stage. 'Paragensis' is not a diagenetic stage; paragenesis is the sequence in which minerals form.

Answer: A

Both (i) and (ii) are true.

(i) Most dune sand is made of quartz grains, often well rounded and frosted by wind abrasion. (ii) Quartz has a hardness of 7 on the Mohs scale and is chemically very stable, so it survives weathering and transport long after feldspars and ferromagnesian minerals have broken down to clay.

In fact (ii) is the reason for (i): because quartz resists both mechanical and chemical breakdown, it is concentrated in mature sediments such as dune and beach sands. Options calling either statement false are therefore wrong. (Gypsum dunes, as at White Sands, and carbonate dunes are exceptions, not the rule.)