Abstract

Lahar facies of the Mount Dutton Formation (Oligocene-Miocene; Marysvale Volcanic Field, Utah) differ in clast-size gradation, presence or absence of a self- supporting clast framework, and, to a lesser extent, thickness. The two most common of these fades (matrix supported and either inversely graded [facies 1A] or nongraded [1B]) are absent from the closest modem analog, Mount St. Helens (MSH), Washington, as are three others (matrix supported normally graded [3], clast supported, inversely graded [5], and clast supported, nongraded [7A]). One common Mount Dutton fades (matrix supported, inversely to normally graded [4]) is the peak-flow overbank and transition lahar fades at MSH. One less common Mount Dutton lahar fades (clast supported, inversely to normally graded [6]) is the sole facies or channel facies at MSH. Mount Dutton facies 2 (thin, matrix supported, nongraded) is present at MSH as the lahar runout facies.

Plots of maximum particle size (MPS) versus bed thickness (BTh) show significant positive correlations for seven fades, as expected of debris-flow deposits. Analogy with MSH lahars suggests that Mount Dutton lahars were cohesionless debris flows (grainflows) that formed when turbulent floods or pyroclastic surges "bulked" by eroding material from their bed. Dispersive pressure produced inverse grading where shearing occurred; inverse grading throughout beds is inherited from transient shearing episodes. Normal grading may be inherited from graded suspensions developed during bulking of turbulent flows or may reflect progressive deposition, rather than frictional or cohesive freezing, of a laterally graded flow.

Slopes and intercepts of correlation lines for individual facies on an MPS-BTh plot form natural groups. Three groups, comprising facies 5 and 7A, 1A and 1B, and 3 and 4, differ chiefly in the intercept of the correlation line with the MPS axis. This intercept indicates thickness-independent strength, the result of cohesion and possibly other factors. All three groups are stronger in this respect, by a factor of 2 to 100, than a sampling of ancient nonvolcanic debris flows. These three groups correspond to diamictite subfacies of nonvolcanic debris flows from the Permian of Colorado.

Facies 6 is unlike other fades of the Mount Dutton Formation but similar to ancient nonvolcanic debris flows in its strength properties. It resembles whaleback bars that formed during nonuniform flow in the North Fork lahars at MSH. Two Mount Dutton fades (2 and 7B [clast supported, normally graded]) do not have a significant positive correlation of MPS and BTh. They are probably deposits of hyper-concentrated fluids, not lahars in the strict sense.

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