Interpretation of Sandstones and Conglomerates

Essay Question

Although conglomerates are but a small part of the sedimentary rock record, they provide an incredible amount of information on sediment source, transport and environment of deposition, and tectonic setting. Consider a wedge-shaped 1000 meter-thick body of extrabasinal, petromict paraconglomerate dominated by pebbles, cobbles and boulders of metamorphic rock fragments, matrix-supported, and lacking any organization to the sandy matrix material.

Interpret the possible source, mode(s) of transport, environment(s) of deposition, and tectonic setting(s) and give reasons for your response.

Multiple Choice Questions

Which one of the following sandstones would be most likely to develop along an active plate margin where subduction is taking place?

a supermature quartz arenite

an immature quartz arenite

a mature feldspathic sandstone

an immature lithic sandstone

A high ratio of (rock fragments + feldspar)/quartz in a sandstone may indicate

deposition on a stable craton

extensive recycling of sediment

derivation from supracrustal source rocks

derivation from subcrustal source rocks

a source dominated by humid climate and low relief

uplift of granitic plutons in source area

A sandstone with a relatively high ratio of quartz to feldspar + rock fragments may indicate

warm moist climate in source

derivation from recycled sandstones

tectonic instablility

tectonic stability

first-cycle sediment transported for short distances and deposited rapidly

Which of the following is likely to be an intraformational conglomerate?

chert pebbles in a sandy matrix dominated by quartz and chert grains

limestone pebble conglomerate with a matrix similar to the clasts

metamorphic rock fragments to boulder size in a matrix of quartz, feldspar and chlorite

mud chips dominated by clay minerals in an immature sandy matrix of quartz and feldspar

pebbles of quartzite in a matrix of quartz sand

matrix supported cobbles and boulders of igneous and metamorphic rocks, lacking internal fabric

Which of the following conglomerates is most likely to form in a shallow-marine high-energy environment in a stable tectonic setting?

petromict orthoconglomerate

petromict paraconglomerate

oligomict orthoconglomerate

oligomict paraconglomerate

limestone pebble conglomerate

tillite