Hemipelagic versus Pelagic Sediments: 24-Pet-A1 Principles of Stratigraphy and Sedimentation, Undated paper, Q7

Question 7 of 19: Hemipelagic versus Pelagic Sediments

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, 2019-May. 3 hours duration; closed book, approved Sharp/Casio 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 6 marks each, for a Part B total of 36 marks – an 86-mark maximum (50 for Part A + 36 for Part B).

Check: the mark values and question count used throughout this solution (6 marks each for Questions 3–19, Part A = 50, Part B = 36, maximum = 86) are as printed on the paper.

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, porosity); Nichols, G., Sedimentology and Stratigraphy, 2nd ed., Wiley-Blackwell (fluvial/deltaic/deep-marine systems, sequence stratigraphy, stratigraphic units); Reading, H.G. (ed.), Sedimentary Environments: Processes, Facies and Stratigraphy, 3rd ed., Blackwell (facies models, alluvial fans, deltas, deep-marine systems); Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (basin analysis, well-log correlation, seismic/acoustic impedance); International Commission on Stratigraphy, International Chronostratigraphic Chart (geological time scale).

Question 7: Hemipelagic versus Pelagic Sediments (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.

Pelagic sediment accumulates in the open ocean, far from any terrigenous source, by the slow settling of biogenic particles (calcareous or siliceous ooze, depending on water depth relative to the carbonate compensation depth) or of fine wind-blown/volcanic dust (abyssal red clay); terrigenous input is negligible and accumulation rates are extremely slow (millimetres per thousand years). Hemipelagic sediment accumulates on continental slopes and rises, closer to a terrigenous source, and is a genuine mixture of fine terrigenous clay/silt (delivered by dilute nepheloid-layer transport, low-density turbidity-current overflow, or wind) together with a pelagic biogenic component; because of the added terrigenous supply, hemipelagic accumulation rates are one to two orders of magnitude faster than pelagic rates (centimetres, not millimetres, per thousand years).

Pelagic versus hemipelagic sediment
Property	Pelagic	Hemipelagic
Setting	Open ocean, far from margin	Continental slope/rise, near margin
Composition	Biogenic ooze or red clay only	Terrigenous mud + biogenic component
Terrigenous input	Negligible	Significant
Accumulation rate	mm/kyr (very slow)	cm/kyr (much faster)

Topic. Pelagic versus hemipelagic deep-marine sediment.

Key relations. Distance from terrigenous source and sediment supply rate control both composition (pure biogenic/eolian vs. terrigenous-biogenic mix) and accumulation rate.

Why this works. Terrigenous mud is only delivered efficiently to sites reasonably close to a continental margin (by nepheloid transport or dilute turbidity-current spillover); once a site is far enough from any margin, the only material reaching it is what falls through the water column itself – biogenic tests/frustules and wind-blown dust.

Common pitfall. Assuming "deep water" alone implies pelagic – a deep continental rise site can still be hemipelagic if it is close enough to a terrigenous sediment source; it is proximity to source, not water depth per se, that is the controlling variable.

Source. Reading (ed.), Sedimentary Environments, 3rd ed., ch.10 (deep-marine sediments); Boggs, Principles of Sedimentology and Stratigraphy, 5th ed., ch.11.