18-Geol-A3 Sedimentation and Stratigraphy · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A3, Sedimentation & Stratigraphy, 2016-May. Closed book, no calculator, 3 hours. Part 1 (Questions 1–12, Sedimentology and Sedimentary Processes) instructs "Answer eight questions of your choice" (8 × 5 = 40 marks) and Part 2 (Questions 13–19, Stratigraphy and Sedimentary Basin Analysis) instructs "Answer five of the following seven questions" (5 × 5 = 25 marks), for a maximum attainable grade of 65/65.
Reference texts: Nichols, Sedimentology and Stratigraphy, 2nd ed. (depositional environments, facies models, flow regime and bedforms, carbonate platforms, sequence stratigraphy, biostratigraphic correlation); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sandstone/carbonate classification, diagenesis, provenance); Selley & Sonnenberg, Elements of Petroleum Geology (turbidite reservoirs, trace-fossil facies context).
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 rimmed shelf has a wave-resistant, high-energy barrier (a reef or oolitic/skeletal sand shoal) along its outer margin that absorbs incoming wave energy, so the platform interior is sheltered and low-energy. This produces a strong energy contrast: coarse, well-washed reefal boundstone/grainstone at the rim, grading shoreward into low-energy, mud-rich lagoonal wackestone/mudstone, often with restricted-circulation facies (evaporites, peloidal mud) in the most protected parts of the lagoon.
A ramp has no abrupt break in slope or rim — energy instead decreases gradually and continuously offshore, from a high-energy, storm- and wave-agitated inner ramp (above fair-weather wave base: skeletal/ooid grainstone, cross-bedded shoals) through a moderate-energy mid ramp (between fair-weather and storm wave base: interbedded grainstone/packstone with storm layers) to a low-energy outer ramp (below storm wave base: fine, undisturbed carbonate mudstone/marl). The facies belts therefore form a smooth, gradational transect rather than the sharp rim-to-lagoon jump of a rimmed shelf.
An isolated platform (an atoll-like build-up surrounded by deep water on all sides, with no attached shoreline) has essentially the entire rim exposed to open-ocean wave energy, so high-energy reefal/grainstone facies can occur all the way around the margin, with only a small, protected central lagoon developing low-energy mud facies if the rim is sufficiently closed. An epeiric (interior) platform is an extremely large, very low-relief, shallow sea flooding a stable craton interior, far from any open-ocean wave fetch; energy is low and uniform across most of its area (fine carbonate mud, microbial/algal-laminated facies), with locally higher-energy shoal facies restricted to gentle topographic highs.
