Geochronostratigraphy versus Geochronology: 24-Pet-A1 Principles of Stratigraphy and Sedimentation, May 2018, Q13

Question 13 of 19: Geochronostratigraphy versus Geochronology

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

EGBC National Exam — Petroleum Engineering, 17-Pet-A1 Principles of Stratigraphy & Sedimentation, 2018-May. 3 hours duration; closed book, no calculator permitted. The paper has two parts: Part A (Questions 1–10, Sedimentology and Sedimentary Processes) – Questions 1 and 2 are mandatory (10 marks each, 20 marks), plus any five of the remaining eight (3–10) at 6 marks each (30 marks), for a Part A total of 50 marks; and Part B (Questions 11–19, Stratigraphy and Sedimentary Basin Analysis) – answer any six of the nine at 5 marks each, for a Part B total of 30 marks – an 80-mark maximum (50 for Part A + 30 for Part B).

Reference texts: Boggs, S. Jr., Principles of Sedimentology and Stratigraphy, 5th ed., Pearson (grain texture, sediment transport, bedforms, carbonate/evaporite systems, sequence stratigraphy, unconformities, stratigraphic principles); Tucker, M.E., Sedimentary Petrology, 3rd ed., Blackwell (sandstone/carbonate classification, diagenesis, dolomitization); Nichols, G., Sedimentology and Stratigraphy, 2nd ed., Wiley-Blackwell (fluvial/deltaic systems, sequence stratigraphy, stratigraphic units); Reading, H.G. (ed.), Sedimentary Environments: Processes, Facies and Stratigraphy, 3rd ed., Blackwell (facies models, deep-marine and shelf systems); Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (source rocks, basin classification, reservoir quality); International Commission on Stratigraphy, International Chronostratigraphic Chart (geological time scale).

Question 13: Geochronostratigraphy versus Geochronology (5 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.

Definitions. A geochronostratigraphic (chronostratigraphic) unit is a body of rock strata formed during a specific interval of geologic time; it is a material (rock) unit, bounded by time-planes, recognized from the rock record itself. A geochronologic unit is the corresponding abstract interval of time itself – not rock, but the span during which the equivalent chronostratigraphic unit was deposited – and exists everywhere, whether or not any rock happens to be preserved from it at a given locality.

Geochronostratigraphic (rock) and corresponding geochronologic (time) ranks, highest to lowest
Geochronostratigraphic unit (rock)	Geochronologic unit (time)
Eonothem	Eon
Erathem	Era
System	Period
Series	Epoch
Stage	Age
Chronozone	Chron

Topic. Geochronostratigraphic (rock) versus geochronologic (time) unit hierarchy.

Key relations. Eonothem/Eon, Erathem/Era, System/Period, Series/Epoch, Stage/Age, Chronozone/Chron – each pair names the same interval, once as rock, once as time.

Why this works. Geologic time exists everywhere continuously, while the rock record is patchy (deposition, non-deposition and erosion vary place to place), so stratigraphy needs two parallel vocabularies: one for the physical rock body actually present, and one for the abstract time interval it represents, usable even where the rock itself is missing.

Common pitfall. Using "Period" and "System" interchangeably – "System" refers only to the rock deposited during that time in a specific place, while "Period" refers to the time interval globally; a section can have an incomplete System for a Period that was, globally, entirely uninterrupted.

Source. Boggs, Principles of Sedimentology and Stratigraphy, 5th ed., ch.15; International Commission on Stratigraphy, International Chronostratigraphic Chart.