Stratigraphic Unconformities: Review of the Concept and Examples from the Middle-Upper Paleozoic

Abstract

The word unconformity was adapted from German geology 3 decades later [4, p. 48] and until the mid- nineteenth century pertained to angular stratal discordances. Awareness of geologic time gaps between parallel bed sets, normally accompanied by signatures of erosion, emerged in late nineteenth century (e.g., [5]) under the influence of Charles Darwin’s conclusion on the principal incompleteness of the stratigraphic record [6]. As most recently reviewed by Miall [1, 7], such stratigraphic breaks between parallel strata were classified into “unconformities Type a” by Blackwelder [8] and shortly after that named disconformities [9]. The other two types of unconformities of Blackwelder [8] were (b) contact between rocks of wholly unlike origin (for example, sandstone resting upon granite); and (c) angular discordance of beds with or without difference in lithologic character. Type (c) is the classical angular unconfor- mity of James Hutton, and type (b) was named nonconformity. The latter term was coined by Pirsson and Schuchert [10] and refined into modern usage by Dunbar and Rodgers [2]. Surfaces between parallel bed sets recording time gaps but not bearing signs of erosion were named paraconformities, as opposed to erosion-marked disconformities [2]. However, the difference between the disconformity and paraconformity more often appears in the ability to recognize erosion and evolves with tools and methods. Here, the term strati- graphic unconformity is used as an equivalent of disconformity. Barrell [11] also coined a term diastem that became adapted for the time value of a sedimentation gap at an uncon- formity. Being most easily identified features, angular unconformities and nonconformities are excluded from further discussion. Disconformity-bounded // Only about 10% of the geologic time is imprinted in sedimentary strata, and the rest is hidden in non-deposition or erosion surfaces called unconformities. Stratigraphic unconformities (disconformities) are principal bounding surfaces in sequence stratigraphy which a geologist would easily identify in the outcrop but dubiously in the subsurface unless core is available. Proportion of mis-identified and overlooked disconformities in subsurface stratigraphy is quite large, which puts a warning sign on simplistic sequence stratigraphic models. Time imprinted in disconformities can be evaluated with relative weathering maturity of the subaerial profile, cyclostratigraphic calibration, absolute dating, and biostratigraphy. However, using biostratigraphy alone is never enough as biostratigraphic gaps tend to fill with increasing data coverage. Identification of paleo-vadose zones and subaerial exposure profiles is regarded critical for finding stratigraphic unconformities and is the only approach in strata where geophysically mappable fluvial systems are absent. Drowning unconformities are carbonate-platform drowning surfaces that usually produce distinct reflection horizons and have better stratigraphic value in the subsurface than platform-embedded subaerial unconformities. This discussion is supported by examples of subaerial disconformities from the Devonian, Carboniferous, and Permian of Canada and Russia and with an example of a geographically extensive mid-Devonian drowning unconformity from Northwestern Canada.