**1. Introduction**

#### **1.1. Definition of the subject**

It is generally accepted that only about 10% of the geologic time is recorded in the sedimentary rocks, whereas 90% is collapsed into non-deposition, alteration, and erosion surfaces collectively called unconformities [1]. Of these diverse surfaces with time value ranging from minutes to hundred millions of years, only those of practical use, that is, traceable on a scale exceeding one outcrop and marked with distinct diagnostic features are discussed below.

#### **1.2. Growth of the concept**

This subheading, borrowed from Dunbar and Rodgers [2], brackets 230 years of unconformity research counting from recognition of an angular unconformity in late 1780s [3]. The word *unconformity* was adapted from German geology 3 decades later [4, p. 48] and until the midnineteenth 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 unconformity* 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 stratigraphic 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 unconformity. Being most easily identified features, angular unconformities and nonconformities are excluded from further discussion.

Disconformity-bounded packages of sedimentary rocks, called cycles, cyclites, cyclothems, allostratigraphic units, and most commonly sequences, remained in focus for many decades, generating an impressive development of concepts, terminology, and discussion on local vs. global controls of base or sea level fluctuations [1, 2, 12–24]. It should be noted that sequence stratigraphy significantly expanded definition of sequences by including both disconformities and their correlative surfaces (conformities) in more complete basincentered sections [14]. Sequence stratigraphy is reviewed in this book but is not the focus of this contribution.
