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24-Pet-A1 Principles of Stratigraphy and Sedimentation · December 2014

Question 16 of 24: Unconformities and Their Types

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

Notes on this paper

EGBC National Exam — Petroleum Engineering, 98-Pet-A1 Principles of Stratigraphy & Sedimentation, 2014-Dec. 3 hours duration; closed book, no calculator permitted. The paper has three parts: Part A1 (Questions 1–7, sedimentary processes, 5 marks each, answer any 4 of 7 for 20 marks), Part A2 (Questions 8–15, brief/multiple-choice, 3 marks each, answer any 5 of 8 for 15 marks), and Part B (Questions 16–24, stratigraphy, 5 marks each, answer any 6 of 9 for 30 marks) – a 65-mark maximum.

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 (carbonate classification, diagenesis, dolomitization); Nichols, G., Sedimentology and Stratigraphy, 2nd ed., Wiley-Blackwell (fluvial systems, sequence stratigraphy, stratigraphic units); Reading, H.G. (ed.), Sedimentary Environments: Processes, Facies and Stratigraphy, 3rd ed., Blackwell (facies models, trace fossils); Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (source rocks, reservoir quality); International Commission on Stratigraphy, International Chronostratigraphic Chart (geological time scale).

Question 16: Unconformities and Their Types (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.

An unconformity is a surface within a stratigraphic succession that represents a significant gap in the geologic record – a period of non-deposition, erosion, or both – separating older rocks below from younger rocks above, such that a measurable interval of time is missing at that surface.

Four types of unconformity Angular tilted beds truncated by flat-lying beds above Disconformity parallel beds, irregular erosional (channelled) surface Nonconformity crystalline basement sedimentary rock over igneous/metamorphic rock Paraconformity flat, undetected surface – gap shown only by dating/fossils
Four unconformity types: angular unconformity (tilted strata truncated below flat strata), disconformity (parallel strata, visible erosional relief), nonconformity (sedimentary rock over crystalline basement), paraconformity (parallel, no visible erosion surface – the gap is recognized only from biostratigraphic/isotopic dating).

The four types differ in the attitude of the beds above/below and in the nature of the surface itself. An angular unconformity separates older strata that were tilted/folded and then eroded from younger strata deposited flat upon that truncated, non-parallel surface. A disconformity separates two parallel sequences of strata (no tilting between them) across a surface with visible erosional relief (channels, karst, relief). A nonconformity separates younger sedimentary strata from older crystalline igneous or metamorphic basement rock. A paraconformity separates two parallel sequences across a surface that shows NO visible physical evidence of erosion or relief at all – the time gap is recognized only indirectly, from missing biostratigraphic zones or radiometric/isotopic age data, making it the most difficult unconformity type to identify in outcrop or core.