HISTORY OF A VOLCANICLASTIC APRON AROUND A MID-TERTIARY TRACHYTE DOME COMPLEX, CENTRAL ARIZONA, USA

The Wells’ Ranch dome field in central Arizona preserves a stratigraphically complex record of the growth of at least five volcanic domes within an area of ~10 km². At least two of the domes were highly explosive, emplacing pyroclastic deposits that built aprons around the domes. Sedimentation was influenced by continuous reorganization of the drainage network caused by apron growth and intrusion of adjacent domes. The resulting geology is a disorganized collage of pre-dome fluvial deposits, trachyte lava flows and breccias, and aprons.

Apron deposits are asymmetrical, thin ribbons ~ 0.1-0.2 km wide around the domes. Thicknesses in complete sections vary from 20 to 60 m. The best-developed apron encompasses an ~135° sector from the NNW through the S of a dome and extends over 2 km from the volcano. The apron is made up of three distinct lobes representing major periods of construction.

The oldest lobe comprises heterolithologic breccia (HB) representing localized, early phases of dome growth. Pyroclastic currents and reworking mixed ash and trachyte clasts with older, mostly fluvial deposits that were uplifted as the dome grew. These deposits dominantly accumulated very close to the dome.

The second lobe comprises surge deposits, monolithologic breccia (MB), and stratified volcaniclastic sandstone mostly WNW of the dome. Deposits thicken and thin in response to interaction with an older lava flow. Clasts are predominately trachyte with rare epiclastic material. Pyroclastic-surge deposits were emplaced on the NW sector of the apron, suggesting northwesterly directed eruptions. Relief on the older breccia lobe prevented accumulation of pyroclastic deposits to the S.

The third lobe spans the entire apron, and probably represents a number of different growth periods. To the S, the basal deposit is a coarse-grained breccia £ 6.0 m thick. The remainder of the lobe in the S, and across the rest of the apron is fine-grained breccia in intervals to 30.3 m thick, thinning distally to 19 m. MB is interbedded with HB, which contains epiclastic sediment only in the sand fraction, or £3% quartz in the sand fraction and £10% epiclastic gravel. Low percentages of epiclastic sediment suggest that these beds result from reworking previous mixed breccia, as well as aeolian sand blown onto the apron and dome flanks from the fluvial system to the west.