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.

摘要

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 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 architectural 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. Mapped accretion surfaces could not be traced either in down dip or strike direction for more than a 101 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. Draping is also discontinuous across architectural surfaces and spatially throughout the point bar. Reservoir connectivity within the point bar depends on the architectural style and is linked to the accretion process. 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.