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18-Geol-A3 Sedimentation and Stratigraphy · May 2015

Question 11 of 18: Formation of Fluvial Point Bars

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

Notes on this paper

EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2015-May. Closed book, 3 hours.

Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate classification, stratigraphic principles and correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone and carbonate classification, diagenesis, evaporites, phosphorites); Selley & Sonnenberg, Elements of Petroleum Geology (reservoir quality, subsurface wireline-log interpretation).

Question 11: Formation of Fluvial Point Bars (10 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.

Point bars form on the inside (convex) bank of a meander bend. Flow rounding a bend develops a helical secondary-flow cell superimposed on the main downstream current: at the water surface, centrifugal effect drives flow OUTWARD toward the concave (cut) bank, while a compensating near-bed return flow moves INWARD toward the convex (point-bar) bank. This helical circulation concentrates erosive energy against the concave bank (undercutting and bank collapse there) while sweeping bedload sediment across the channel floor toward the convex bank, where it is deposited as the channel migrates laterally through time — a process called lateral accretion.

Meander bend: point bar & cut bank (plan view) Point bar (convex bank) deposition — lateral accretion Cut bank (concave) erosion, bank collapse helical flow (schematic) fining-upward channel-lag / trough x-bed sand ripple-laminated fine sand overbank mud (epsilon x-bed tops)
Point-bar deposition on the convex bank opposite an eroding cut bank, with the resulting fining-upward internal sequence (epsilon/lateral-accretion cross-bedding).

Internally, a point bar therefore records a characteristic fining-upward sequence: a basal channel-lag or coarse trough cross-bedded sand grades up into ripple cross-laminated fine sand and, at the top, into overbank fines, with the whole succession cut by low-angle, inclined epsilon (lateral-accretion) cross-bedding surfaces — each surface recording one increment of the channel's lateral migration across the floodplain. Because the point bar grows at essentially the same rate the opposite cut bank retreats, the channel BELT itself does not widen through time; it simply translates sideways across the floodplain, reworking its own earlier overbank deposits as it migrates, which is why ancient point-bar sand bodies are commonly preserved as laterally extensive, ribbon-like sheet sands rather than as narrow, isolated channel fills.