18-Geol-A3 Sedimentation and Stratigraphy · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2017-Dec. Closed book, 3 hours, no calculator permitted.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate platforms, stratigraphic principles, correlation, sequence stratigraphy); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (weathering, sandstone and carbonate classification, diagenesis); Allen & Allen, Basin Analysis, 3rd ed. (basin classification, subsurface mapping); Tearpock & Bischke, Applied Subsurface Geological Mapping, 2nd ed. (structure-contour, isopach and lithofacies mapping, syndepositional structures).
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 depositional sequence is a relatively conformable succession of genetically related strata, bounded above and below by unconformities (and their laterally correlative conformities). It is the fundamental building-block unit of sequence stratigraphy, generated by one complete cycle of relative sea-level change acting on a basin's available sediment supply.
Within a sequence, three system tracts are recognized, each tied to a distinct segment of the relative sea-level curve. The Lowstand Systems Tract (LST) is deposited immediately above the sequence boundary while relative sea level is at or near its lowest point and beginning to rise slowly, producing incised-valley fill on the exposed shelf and basin-floor/slope fans from sediment bypassing it. The Transgressive Systems Tract (TST) is deposited as the rate of relative sea-level rise accelerates and outpaces sediment supply, producing a retrogradational (landward-stepping) parasequence stack, and is capped by the maximum flooding surface (MFS) — the point of maximum landward facies shift and minimum sediment supply, often marked by a condensed section. The Highstand Systems Tract (HST) is deposited above the MFS as the rate of relative sea-level rise decelerates and sediment supply again outpaces the (slowing) creation of accommodation space, producing an aggradational-to-progradational (seaward-building) stack that continues to the next sequence boundary.
The direct link to sea-level change is that each system tract's internal stacking pattern is set entirely by the balance between the rate of relative sea-level rise (which creates accommodation space) and the rate of sediment supply: LST forms when accommodation is barely being created (sea level near its lowest); TST forms when accommodation creation outpaces supply (retrogradation); HST forms when supply again outpaces a slowing rate of accommodation creation (progradation). Because the system tracts are a direct readout of the sea-level curve's shape, a well-dated sequence-stratigraphic section can itself be used to reconstruct the relative sea-level history that produced it.
