Interpretation of Bounding Surfaces within Tidal Deposits: How Much Can We Reduce the “Fog Index”?

Yoshida, Shuji¹, Ron J. Steel², Robert Dalrymple³, James MacEachern⁴, Kerrie Bann⁵ (1) Chiba University, Chiba, Japan (2) The University of Texas at Austin, Austin, TX (3) Queen's University, Kingston, ON (4) Simon Fraser University, Burnaby, BC (5) Ichnofacies Pty. Ltd, Calgary, AB

Tidal deposits can be viewed in a hierarchy of scales, ranging from basin (via sequence stratigraphy), through architectural elements, facies, to the sub-facies (laminae, grain) level. Reservoir analogue studies commonly focus on the external and internal geometric arrangements of tidal sand bodies, and the distributions of discrete facies and their assemblagestherein. In outcrops, systematic vertical changes in the type and distribution (dimension, interconnectedness) of channels/bars may be recognized at the sequence-stratigraphic scale. Such changes, however, reflect not only variations in Accommodation/Sediment-supply ratios or other allocyclic controls, but autocyclic processes, such as filling of a channel or bay, as well.

It is at the facies scale that we first recognize tidal deposits from cores or outcrops, and the larger framework must be built from the bottom up. Delineation of bounding discontinuities (bar boundaries, sequence boundaries, etc.) is commonly difficult, however, particularly in low-accommodation regimes, and this impedes paleogeographic interpretations of the setting. In outcrops, this difficulty arises mainly from: (i) lateral facies variations across distances as short as 10's m; (ii) scattered occurrence of environmental indicators; and (iii) recent weathering effects. Nonetheless, combining some facies indicators is useful in helping to establish a sequence-stratigraphic framework. Transgressive estuarine (as opposed to regressive deltaic) settings, for example, are characterized by upward increases in salinity (manifest by changes in trace-fossil suites), occurrence of thick (c. 1-10 cm) fluid-mud layers, and areal extent of bedforms. A variety of outcrop examples, including the Upper Cretaceous Sego Sandstone in Utah, serve to illustrate these points.