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The existing hierarchy of formal lithostratigraphic units of the North American Stratigraphic Code (NACSN 2005) recognizes “Bed” or “Beds” (Flow in the case of volcanic rocks) as the only formal subdivision of members. Beds, in a geological context, are discrete depositional/accumulation units bounded by bedding planes and the formal term Bed should not be applied to thicker subdivisions of members composed of several discrete and distinguishable beds, especially of differing lithology. In such cases, thicker divisions of members that may contain one or more formally named Beds, would be usefully assigned to the division termed “Submember”. In this paper we review the issue, discuss the justification of formalizing submembers, and propose a formal amendment to the North American Stratigraphic Code.
There are several advantages to formalizing this additional division. First, submembers are already being used informally in many units. Formalization of the concept would require that these divisions meet the same standards as other stratigraphic units, e.g., designated
type sections and careful specification of boundaries. Second, the formal use of Submembers would avoid the confusing issue of having a formally named “Bed” within an interval termed “Beds”. Third, the addition of an intermediate rank permits accurate communication and precise correlation of complex sedimentary successions. The insertion of the rank of submember would allow retention of existing formal members and beds, rather than promoting the addition of new members to replace overly thick intervals formally named beds