Origin and Scaling of Surfaces and Heterogeneity in a Fragmentary Point Bar From 3-D Architectural-Element Analysis, Late Cretaceous Dinosaur Park Formation Within Dinosaur Provincial Park, Alberta, Canada

Abstract

Point bar deposits are intrinsic architectural elements of meandering rivers. Traditional models commonly record point bar preservation as continuous accretionary bodies with continuous bounding surfaces that extend along the entire bar face. Although preservation of point bars in this fashion is common, fragmentary bar accretion, whereby accretionary units are preserved as incomplete and dispersed fragments, is also found in both ancient and modern river deposits. Accretionary events in the proposed model are regular and frequent, but are consistently eroded and reworked by subsequent accretion events leading to a complex fragmental architecture. This process generates a complex internal architecture and internal heterogeneity recorded by a hierarchy of bounding surfaces. This hierarchy is documented by a 3D architectural-element analysis on a Terrestrial Laser Scanning (TLS) model of a 9m point bar in the Late Cretaceous Dinosaur Park Formation within the Steveville area of Dinosaur Provincial Park, Alberta, Canada. Four distinct architectural orders make up the Steveville point bar. 1st order surfaces record migration of dune bodies. 2nd order surfaces record preservation of individual unit bars during the waning stages of a flood. Unit bars are typically the lowest order architectural element that is commonly preserved along the point bar. Lower medium to fine sand unit bars as seen in this point bar fine upward vertically and are commonly draped by siltstones. Geometry of unit bars and drapes are discontinuous both in strike and dip view. 3rd order surfaces record major flood events that scour into the underlying deposits and set the base for unit bars to build upon. 4th order surfaces record a large reorientation in point bar accretion owing to allocyclic changes in the system and a shift from sandy to muddy deposition. Lastly, 5th order surfaces bind the point bar story and record point bar cutoff. The dominant autocyclic process driving the fragmentary nature of the point bar include changes in the pitch between 3rd order accretionary events allowing for the accretionary body to scour into the one beneath and forcing an alternation of slope in 3rd order surface sets. A change in flow regime of the point bar overtime leads to a change in pitch of accretionary bodies. This reworking because of change in pitch contributes to the “Sadler Effect” where long-term bar accretion rates appear slower than short-term accretion rates.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016