18-Geol-A3 Sedimentation and Stratigraphy · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2018-May. Closed book, no calculator, 3 hours, 80 marks total. Part 1 (Questions 1–10, Sedimentology and Sedimentary Processes, 50 marks) instructs "Questions 1 and 2 must be answered (10 points each); answer any FIVE of Questions 3–10 (6 points each)." Part 2 (Questions 11–19, Stratigraphy, 30 marks) instructs "Answer any FIVE of Questions 11 to 19 (6 points each)."
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, sequence stratigraphy, biostratigraphy and correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone and carbonate classification, weathering, diagenesis); Bjorlykke, Petroleum Geoscience: From Sedimentary Environments to Rock Physics, 2nd ed. (basin settings, reservoir facies).
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.
The fundamental unit of biostratigraphy is the biozone (biostratigraphic zone) — a body of rock defined and characterized by its fossil content, distinct from the rock units above and below it.
Biozones are established in several ways, each defined by a different criterion: a range zone defines the interval between the first and last recorded occurrence (FAD/LAD) of a single chosen taxon; a concurrent-range zone defines the interval where the ranges of two or more taxa overlap; an assemblage zone is defined by a distinctive natural association (assemblage) of three or more taxa found together, regardless of their individual total ranges; an interval zone is simply the rock between two defined biohorizons (e.g. between the LAD of one taxon and the FAD of another), without reference to what occurs within it; and an abundance (acme) zone is defined by the interval where a taxon reaches its peak relative abundance, rather than by its presence/absence alone.
All of these differ from purely lithologic units (formations) because biozone boundaries are drawn strictly on fossil first/last appearances or assemblage changes, which — because biological evolution is time-progressive and (for well-chosen taxa) not tied to a particular facies — allows biozones to be correlated over wide areas independent of local lithology, making them the primary tool for relative age correlation between basins.
