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 rimmed shelf has a wave-resistant, high-energy barrier (a reef or an oolitic/skeletal sand shoal) along its outer margin that absorbs incoming wave energy, sheltering the platform interior. This produces a sharp energy contrast: coarse, well-washed reefal boundstone/grainstone at the rim, grading shoreward into low-energy, mud-rich lagoonal wackestone/mudstone, with restricted-circulation facies (evaporites, peloidal mud) in the most sheltered part of the lagoon.
A ramp has no abrupt break in slope or rim, so energy instead decreases gradually offshore — from a high-energy, storm/wave-agitated inner ramp (above fair-weather wave base: ooid/skeletal grainstone shoals) through a moderate-energy mid ramp (storm-influenced packstone/grainstone) to a low-energy outer ramp (below storm wave base: fine, undisturbed carbonate mudstone/marl). The facies form a smooth gradational transect rather than the rimmed shelf's sharp rim-to-lagoon jump.
An isolated platform (an atoll-like build-up surrounded by deep water on all sides, with no attached shoreline) has essentially its entire margin exposed to open-ocean wave energy, so high-energy reefal/grainstone facies can develop all the way around the rim, with only a small, protected central lagoon 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 open-ocean fetch; energy is low and relatively uniform across most of its area (fine carbonate mud, microbial/algal-laminated facies), with locally higher-energy shoal facies restricted to gentle topographic highs.
