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.
The framework grains are the load-bearing, sand-size (0.0625–2 mm) particles that form the rock's skeleton: quartz (monocrystalline and polycrystalline), feldspar (K-feldspar and plagioclase), and lithic (rock) fragments. The matrix is the fine-grained (<0.03 mm, clay- and silt-size) material occupying the space between framework grains, deposited at the same time as the sand fraction (primary detrital mud). The two most common cement minerals precipitated later from pore fluids to bind the grains together are quartz (silica overgrowths) and calcite.
Six framework classes result from the Q–F–L triangle. A quartz arenite (>90% Q) is an ultra-mature sand composed almost entirely of stable, resistant quartz grains, indicating intense weathering and/or prolonged transport/recycling. Between 75% and 90% Q, a sand with more feldspar than lithics is a subarkose, and one with more lithics than feldspar is a sublitharenite. Below 75% Q, a feldspar-dominated sand (F > L) is an arkose — typically sourced from rapid, short-transport erosion of a granitic/gneissic terrane, often in an arid or tectonically active setting that limits chemical breakdown of feldspar — while a lithic-fragment-dominated sand (L > F) is a litharenite, sourced from erosion of fine-grained volcanic, sedimentary or low-grade metamorphic rock. Any of these six classes additionally carries a wacke prefix/suffix (e.g. lithic wacke, feldspathic wacke) when matrix content is 15–75%, distinguishing a poorly sorted, rapidly-deposited sand from its clean, winnowed arenite counterpart (<15% matrix).
