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 paleosol is a soil horizon that developed at a former (now buried) land surface during a period of subaerial exposure and non-deposition, and was subsequently preserved by burial beneath younger sediment. It is a direct stratigraphic record of a depositional hiatus long enough for pedogenic (soil-forming) processes to act on the exposed substrate, and paleosols are therefore widely used to identify sequence boundaries and other unconformities in the rock record.
A paleosol is recognized in outcrop or core from a combination of pedogenic features that are not produced by normal sedimentary deposition: soil horizonation (a vertically differentiated A–B–C profile, with the original bedding of the parent material progressively destroyed upward); root traces and root casts/molds, which physically prove vegetation grew in place; illuvial clay coatings (cutans) lining former root channels and peds; pedogenic carbonate nodules or a caliche/calcrete horizon in arid-to-semiarid settings, or a lateritic/bauxitic profile in humid tropical settings; mottling (irregular reddish and greyish-green patches from fluctuating redox conditions in the vadose zone); and a characteristic reddish colouration from in-situ iron oxidation. Contact relationships help confirm the interpretation: the base of a paleosol typically grades gradually into unaltered parent material below, while its top is commonly sharp and erosional, or is onlapped abruptly by the overlying, unrelated sedimentary unit.
