Inclined heterolithic stratification—terminology, description, interpretation and significance

Abstract

Parallel to sub-parallel strata possessing original (depositional) dips occur within both lithologically “homogeneous” and “heterogeneous” units of water-lain, siliciclastic sedimentary sequences. Most such inclined strata form as a result of the lateral growth of “active”, large-scale “bedforms” such as point bars or Gilbert-type deltas. The confusing diversity of terms previously used to describe inclined stratified deposits is reviewed. Virtually all these terms, including epsilon-cross-stratification and its derivatives are unsatisfactory because they are non-descriptive and/or communicate an overt genetic bias. The names Inclined Heterolithic Stratification (IHS) and Inclined Stratification (IS) are proposed as replacements. To facilitate comparison of IHS deposits a “standard” descriptive nomenclature is also proposed. IHS may occur as solitary sets or show vertical or lateral stacking forming cosets . Co-directional laterally stacked sets constitute an imbricate coset. Composite sets are those in which IHS sequences gradationally overlie inclined-stratified lithofacies units (typically sandstones). Individual inclined units comprising IHS sets may be either normally graded or (more commonly) consist of two distinct lithological members ar anged as a coarse-to-fine couplet . Inclined units are separated by inclined surfaces indicative of non-deposition or erosion. Published examples of modern and ancient IHS deposits are known or inferred to occur in a variety of environments, but the overwhelming majority are products of point-bar lateral accretion within meandering channels of freshwater rivers, tidally influenced rivers and creeks draining intertidal mudflats. Descriptions are given of the most characteristic and important (from an interpretation standpoint) physical features of point-bar IHS deposits and their probable modes of origin. Deposits predominantly composed of sand and mud layers arranged as coarse-to-fine couplets are emphasized. Factors thought to control the formation and preservation of sand-mud couplets in the tidally influenced river point-bar depositional environment are described and their probable effects evaluated. Several potentially useful criteria for the differentiation of ancient freshwater versus tidally influenced river point-bar IHS sequences are discussed. The significance of IHS deposits for: (1) palaeoenvironmental and palaeogeographic interpretation; (2) reconstruction of palaeochannel morphological characteristics; and (3) economic geology is outlined. Future recognition of tidally influenced river point-bar IHS in the rock record should furnish valuable information regarding shoreline proximity, possible palaeotidal ranges etc. Much additional work is required on IHS deposits of modern point bars in general, and tidally influenced river point bars in particular, before satisfactory process-response depositional models of their formation can be developed.