UNCONFORMITIES: BUILDING GEOLOGIC KNOWLEDGE FROM “MISSING” TIME

What can we learn from what’s not there? Undergraduate geology students practice using stratigraphic principles such as superposition and cross-cutting relationships to interpret the sequence of geologic events that formed the rock layers within a stratigraphic section. They learn that unconformities are surfaces between strata of distinctly different ages – that along an unconformity a significant amount of time is “missing” or unrepresented in the rock record. They learn to recognize some of the clues that an unconformity may exist, such as truncation of underlying strata or structures; basal conglomeratic or fossiliferous lags above an unconformity; differential mineralization, cementation, weathering, or soil development at an unconformity; or gaps in faunal succession or facies succession across an unconformity. They learn to distinguish between different types of unconformities – angular unconformities, nonconformities, disconformities, and paraconformities – and to interpret some causative geologic events such as uplift and erosion. In general, however, the proportion and significance of geologic time that is unrepresented in the rock record remains unclear to many students, as does the scope of information that can be gleaned from unconformable surfaces. Guiding students to deeper investigations of the “how and why” behind unconformities promotes understanding of valuable geologic concepts such as: visualizing the movement of geologic conditions and processes through three spatial dimensions over the fourth dimension of time; the importance of “looking” up, down, laterally, and from local to regional scales when interpreting a geologic section; and remembering that what we observe directly in the rock record likely represents a statistically unusual combination of natural conditions rather than “average” ones. Students also learn to use modern geologic analogs to help evaluate the validity of reconstructions of what occurred during an interval of time that is unrepresented in the rock record.