New Mexico Geological Society Annual Spring Meeting — Abstracts

Calcite cemented zones in the Miocene Zia Formation of the lower Santa Fe Group exhibit a I wide range of morphologies, from isolated oval and elongate shapes to laterally extensive sheets (>1km). Excellent exposures allow detailed examination of geometric relationships between primary textures, cementation, and depositional environment. Laterally extensive cemented zones are calcretes formed predominately in mixed fluvial-eolian environments during periods of depositional stability and soil formation. The formation of small lenticular, ovoid or elongate
(oriented) concretions is controlled by primary permeability in the sands (i.e. they form preferentially in coarser and better sorted layers). These smaller concretions are more common in strictly fluvial or eolian environments(i.e. unmixed environments). The lower part of the Zia (Piedra Parada and Chamisa Mesa Members) is largely eolian in character, and concretionary zones I are small lenticular bodies and isolated nodules. The middle Zia (Canada Pillares Member) is of fluvial origin, and concretions are much the same. The upper Zia (Unnamed Member) has many laterally extensive concretionary zones(calcretes), as well as many smaller and isolated concretions. Laterally continuous cemented zones can slow or stop vertical fluid flow. Smaller less continuously cemented zones have little or no effect on vertical fluid flow. Oriented concretions may be important as paleo-groundwater flow indicators.