FIELD AND PALEOMAGNETIC CHARACTERIZATION OF LITHIC AND SCORIACEOUS BRECCIAS AT PLEISTOCENE BROKEN TOP VOLCANO, OREGON CASCADES

Cirque-wall exposures of cone-forming deposits of Pleistocene Broken T op volcano, Oregon Cascade Range, reveal that the volcano is composed of unconformity-bounded constructional units of coherent lava (lava-fl ow cores) and breccia. Coarse-grained autoclastic breccias are found a bove and below lava-flow cores and may extend downslope from coherent lava outcrops where they may or may not be associated with thin lava s tringers. Mantle-bedded scoria-fall breccias are recognized by general ly good sorting, mantle bedding, and presence of aerodynamically shape d bombs. These breccias vary considerably in thermal oxidation colorat ion (black, red, orange, purple). Many breccia layers are unsorted mix tures of scoria and lithic (nonvesicular) fragments that grade lateral ly to unambiguous autoclastic breccia or lava-flow cores. These layers are interpreted as hybrid pyroclastic-autoclastic deposits produced b y incorporation of falling or fallen tephra into advancing lava-flow f ronts. This latter breccia type is common at Broken Top and offers par ticular challenges for clast or deposit classification. Progressive th ermal demagnetization results for selected examples of different brecc ia types show that most scoria-fall and autoclastic breccias are empla ced at elevated temperatures (averaging 100-300 degrees C). Clasts wit hin single deposits record different emplacement temperatures ranging, in some cases, from 100 to over 580 degrees C indicating a lack of th ermal equilibration within deposits. Magnetization directions for sing le breccia deposits are more dispersed than data typically reported fo r lava flows. Settling and rotation of clasts after cooling or incorpo ration of colder clasts that are not significantly reheated probably a ccounts for the relatively high dispersion and suggests that paleomagn etic studies demanding low within-site dispersion (e.g., for determini ng paleomagnetic poles or evaluating tectonic rotation) should avoid v olcanic breccias. (C) 1998 Elsevier Science B.V. All rights reserved.