18-Geol-A3 Sedimentation and Stratigraphy · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
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 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.
Phosphorites are sedimentary rocks enriched in phosphate minerals — predominantly carbonate-fluorapatite ("francolite") — typically containing ≥~18–20% P2O5, occurring as nodules, peloids, laminated crusts or replaced skeletal/carbonate grains, most often within marine shelf and upper-slope successions.
Their origin begins with coastal or shelf-margin upwelling: wind-driven divergence at the ocean surface draws cold, nutrient- and phosphate-rich deep water up onto the shelf. The elevated phosphate fuels intense surface biological productivity; as the resulting organic matter (plankton, fecal pellets) sinks and decays in the sediment and pore waters, it releases dissolved phosphate that locally raises pore-water phosphate concentration above the saturation threshold for carbonate-fluorapatite. Apatite then precipitates authigenically — directly as micro-crystalline cement/peloids, or by REPLACING pre-existing carbonate mud and skeletal grains (bioclasts, ooids) grain-for-grain while preserving their original texture. Because phosphogenesis is a slow, authigenic process, phosphorite deposits are strongly associated with condensed, sediment-starved intervals — winnowing by bottom currents concurrently removes fine mud and concentrates the dense phosphatic grains into a residual lag, and periods of low net terrigenous/carbonate sediment supply give authigenic phosphate mineralization time to develop before burial dilutes it.
Economically significant phosphorite belts — the Permian Phosphoria Formation of the western United States, the Neogene phosphorites offshore Peru and Chile, and the Cretaceous–Eocene phosphorites of Morocco and the broader Tethyan margin — all occupy the same upwelling-shelf-margin paleogeographic setting, which is why phosphorite exploration leans as heavily on PALEOCEANOGRAPHIC reconstruction (identifying, from plate-tectonic paleogeography, which ancient shelf margins once faced into a persistent wind-driven upwelling cell) as on conventional facies mapping.
