New Mexico Geological Society Annual Spring Meeting — Abstracts

Directions of thermoremanent magnetizations (TRMs) have been used to correlate and identify ignimbrites (ash flow tuffs) and related units in the Steeple Rock Mining District of southwestern New Mexico. These units have been difficult to correlate by lithology and geologic mapping, due to varying degrees of alteration (acid sulfate, propylitic to sericitic, and silification) and structural complexity in the area. The ignimbrites of primary interest in this study are the Bloodgood Canyon Tuff, and a set of younger tuffs, including the tuff of Crom Ranch and at least three additional ignimbrites.

Over 80% of the 30 sites collected exhibited well grouped (alpha95<12°) TRM directions that are primarily carried by finely dispersed assemblages of magnetite and hematite, as determined by alternating field demagnetization data. Approximately 80% of 30 sites sampled also displayed secondary remanent magnetizations that are due to lightning induced isothermal remanent magnitizations ( IRMs ) and chemical remanent magnetizations ( CRMs ) related to the regional alteration. The CRMs occur in 67% of the sites sampled and are easily identified by their removal at low alternating field demagnitization levels, similar to lightening induced IRMs. Unlike lightning induced IRMs, which produce widely dispersed directions, the alteration CRMs are characterized by well grouped alpha95s as low as 6.2°.

Nineteen of the sites drilled produced remanance directions that were useful for correlation purposes. The Bloodgood Canyon Tuff is easily identified by its reversed direction (dec=158.8°, inc= -29.2°, a95= 9.6°, n= 5), which is carried by fine grained hematite. The younger tuffs consist of a sequence of three closely erupted ignimbrites that have well defined, distinct, normal remanence directions (tuff 1: dec= 332.7°, inc= 58.5°, a95= 8.9°, n= 5; tuff 2: dec= 23.3°,inc=53.4°, a95= 16.6°, n= 4; tuff 3: dec= 356.9°, inc= 74.4°, a95= 2.9°, n= 1). The TRMs in two younger tuffs are carried by finely dispersed magnetite, while the TRM of the third younger tuff is carried by hematite. The remaining younger ignimbrite, the tuff of Crom Ranch, had a reversed direction (dec= 166.7°, inc= -11.9°, a95= 16.0°, n= 4) carried by fine grained hematite. The remaining six sites drilled yielded usuable values of alpha95 which did not correlate with any of the known ignimbrites of the region. These data may reflect either errors in structural orientations or unidentified ignimbrites related to local rhyolitic dome building.