A High-Resolution 3-D Architecture of a Cretaceous Point Bar Using Terrestrial Laser Scanning of Multiple Exposures: A Far More Complex Model of Bar Growth at the Scale of a Steam Chamber Than Previously Thought

Abstract

We produced a 3D Terrestrial Laser Scanning (TLS) model of strata of a single 9m thick point bar in the Late Cretaceous Dinosaur Park Formation of the Steveville area of Dinosaur Provincial Park, Alberta, Canada. The highly dissected landscape and sparse vegetation in the study area provide optimal conditions to observe bar internal architecture in three dimensions from TLS data, permitting us to complete a 3D architectural-element analysis of a 200m by 150m cube in a single point bar. The goal of this 3D fine scale architectural-element analysis is to add increased understanding of the connectivity within point bars at the scale of a steam chamber through better constraint on the distribution of lithologies and surfaces. Lithologies of this point bar record a fining upward trend from basal lower medium to fine sand, upward into siltstone, mudstone, as well as peaty clay on the bar top. The base is also more homogeneous, consisting mainly of lower medium sand with some silt drapes. Heterolithics increase with the general fining upward trend and include siltstones and mudstones. Mapped accretion surfaces could not be traced either in down dip or strike direction for more than a few meters before being truncated by younger accretion events. Point bar deposits consist of centimeter to meter vertical-scaled and imbricately stacked unit bars. Consecutive unit bars are commonly reshaped by erosion and/or depositionally draped. This highly fragmented accretion style results in a lack of accretion surfaces that span the length or height of the full point bar. Reservoir connectivity within the point bar depends on the architectural style and is linked to the accretion process. These bars accreted by deposition and reshaping of small bar fragments that were commonly reworked, resulting in a highly fragmented and heterogeneous architecture. This differs from models of point bars that result from large sheets that accumulate across accretion surfaces producing larger discrete reservoir compartments. Bars of the type identified here are identified in the modern Powder River and result from a regime of frequent irregular floods. The high level of reworking and localized accretion noted for this bar is consistent with other observations from modern bars that report local short term accretion during point bar growth to be an order of magnitude slower than long-term average point bar growth rates.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015