Abstract: Terrestrial Impact Structures: Basic Characteristics as a Guide to Recognition and Exploration

Richard A. F. Grieve

Over 140 terrestrial impact structures, (approx.) 30% of which are buried, are currently known. There are two types of crater forms: simple structures, which consist of a bowl-shaped depression partially filled with allochthonous breccia and having a structurally uplifted and locally overturned rim; and complex structures with an uplifted central area, manifested as a central peak and/or rings, an annular down-dropped trough and a faulted rim area. Complex structures occur at diameters >2 km and >4 km in sedimentary and crystalline targets, respectively, and represent a highly modified crater form related to changes in cratering mechanics with increasing impact energy. As the physics of cratering are, to a first order, independent of target rock characteristics impact structures have relatively fixed morphometric relations as a function of diameter (e.g., the maximum amount of stratigraphic uplift in the center of complex structures is 1/10 the rim diameter). Confirmation of an impact origin requires the recognition of shock metamorphic effects. The exact nature of the resultant impact lithologies (e.g., breccias, impact melt rocks, etc.) is, in part, a function of target rock type, but these variations are well understood. A number of impact structures produce hydrocarbons, in some cases with high flow rates due to brecciation and fracturing. Impact structures may also act against oil migration. Recognizing that a structure is impact produced, however, is a necessary first step in defining a rational strategy for exploration.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994