Question 5 of 21: Question 2, Part 3: Sedimentary Rock Identification

Question 2, Part 3: Sedimentary Rock Identification (6 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

a) Conglomerate. The photo shows coarse, mixed angular-to-rounded clasts (gravel to cobble sized) set in a finer sand/mud matrix – conglomerate forms when gravel-sized clastic sediment, transported by high-energy water (rivers, alluvial fans, beaches), is deposited, buried, and lithified by compaction and cementation; the mix of rounding suggests short-to-moderate transport distance from a variety of parent-rock sources.

b) Coquina (bioclastic/fossiliferous limestone). A rock built almost entirely of loosely cemented fossil-fragment debris is coquina, a variety of limestone: it forms in shallow, high-energy marine or lagoonal settings where wave/current action breaks and concentrates fossil debris, which is then weakly cemented by calcite precipitating from seawater, leaving the rock's characteristic high porosity.

c) (Sedimentary) Breccia. Angular clasts (unlike a conglomerate's rounded clasts) set randomly in a finer matrix indicate very little transport before burial – breccia forms from broken rock fragments (e.g. talus/scree at the base of a cliff, or fault/collapse debris) that are buried and cemented close to their source, preserving their sharp, unabraded edges.