The application of cone penetration test data to facies analysis of the Fraser River Delta, British Columbia




Monahan, Patrick A.

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Cone penetration tests (CPTs) have been developed for engineering investigations of sands and finer sediments. CPTs produce high resolution, repeatable and continuous records to depths of several tens of metres, and resemble wireline logs used in the petroleum industry. It is the objective of this dissertation to demonstrate that they can be used for facies analysis in a similar manner, by using these data to develop a facies model for the modern Fraser River delta, British Columbia, Canada. CPT data provide reliable estimates of sediment type and gram size, so that bed thicknesses, sharp and gradational contacts, coarsening and fining upward sequences, bed continuity and dips can be readily identified. The facies model of the Fraser delta is based on a database of over 800 CPTs and 20 continuously cored boreholes. These data demonstrate that the topset is dominated by a nearly continuous sharp-based sand unit that is 8 to 30 m thick, fines upward and is interpreted to represent a complex of distributary channel deposits. The widespread distribution of this sand unit is the result of distributary channel migration in a tidal flat setting and avulsion or channel switching in the upper delta plain. The sand unit is gradationally overlain by a thinner sequence of interbedded sands and silts deposited in tidal flat, abandoned channel and floodplain environments. Deposits of the upper foreset (<60 m) dip up to 7° seaward and are dominated by silts, interbedded and interlaminated with sands. Several intergradational facies, ranging from dominantly silt to dominantly sand, occur and represent increasing proximity to active distributary mouths. These sediments are organized into metre-scale sandy and silty coarsening-upward sequences that are interpreted to represent annual deposits, and sharp-based sand units that represent sedimentary gravity flow deposits. Deeper foreset deposits are dominated by bioturbated silts. The distribution of facies on both the topset and the foreset has been controlled by the interaction of tidal and fluvial processes. CPT data played a key role in developing this facies model of the Fraser River delta. Most facies have distinct CPT signatures. For example, the topset sand unit and overlying deposits have a CPT signature comparable to the “bell-shaped” gamma ray log signature typical of channel deposits. In the foreset, the seaward dips, the coarsening upward sequences and the sharp-based sands are readily observable on CPT data. Although cores were essential to confirm the facies significance of these signatures, the large volume of CPT data permitted recognition of facies distributions and relationships “at a glance” throughout the delta, rather than at the relatively few site where continuous cores were available. Furthermore, CPTs can be acquired for a fraction of the cost of continuous cores, so that CPT data are potentially an invaluable tool for stratigraphic investigations of other modern sedimentary environments dominated by sands and finer sediments.



Facies (Geology), Sedimentation and deposition