Geologic Map of Lassen Volcanic National Park and Vicinity, California By Michael A. Clynne and L.J. Patrick Muffler Pamphlet to accompany Scientific Investigations Map 2899 Lassen Peak and the Devastated Area Aerial view of Lassen Peak and the proximal Devastated Area looking south. Area with sparse trees marks the paths of the avalanche and debris-flow deposits of May 19–20, 1915 (unitsw9 ) and the pyroclastic-flow and fluid debris-flow deposits of May 22, 1915 (unit pw2) (Clynne and others, 1999; Christiansen and others, 2002). Small dark crags just to right of the summit are remnants of the May 19–20, 1915, lava flow (unitd9 ). The composite dacite dome of Lassen Peak (unit dl, 27±1 ka) dominates the upper part of the view. Lithic pyroclastic-flow deposit (unitpfl ) from partial collapse of the dome of Lassen Peak is exposed in the canyon of the headwaters of Lost Creek in center of view. Ridges flanking central area are glacial moraines (unitQta ) thinly covered by deposits of the 1915 eruption of Lassen Peak (Christiansen and others, 2002). Small permanent snowfield is seen on the left lower slope of Lassen Peak. Area east of the snowfield is the rhyodacite lava flow of Kings Creek (unitrk , 35±1 ka, part of the Eagle Peak sequence). Dacite domes of Bumpass Mountain (unit db, 232±8 ka), Crescent Crater (unit dc, 236±1 ka), hill 8283 (unit d82, 261±5 ka), and Loomis Peak (unit rlm, ~300 ka) are part of the Bumpass sequence. Photograph by Michael A. Clynne. 2010 U.S. Department of the Interior U.S. Geological Survey This page intentionally left blank CONTENTS Introduction.....................................................................................................................................................1 Regional Volcanism .......................................................................................................................................1 Calc-Alkaline Volcanoes ......................................................................................................................1 Low-Potassium Olivine Tholeiite (LKOT) ...........................................................................................2 Volcanic Centers ............................................................................................................................................2 Tectonic Setting .............................................................................................................................................3 Eruptive History of the Lassen Area ...........................................................................................................4 Regional Volcanic Rocks .....................................................................................................................4 Pliocene-Early Pleistocene Volcanic Centers .................................................................................4 Dittmar Volcanic Center ..............................................................................................................5 Maidu Volcanic Center ...............................................................................................................5 Lassen Volcanic Center .......................................................................................................................6 Rockland Caldera Complex ........................................................................................................6 Brokeoff Volcano .........................................................................................................................7 Lassen Domefield.........................................................................................................................7 Holocene Activity ................................................................................................................8 Chaos Crags ................................................................................................................8 Cinder Cone ................................................................................................................8 1914-1917 Eruption of Lassen Peak .........................................................................9 Caribou Volcanic Field .......................................................................................................................10 Red Cinder Chain .......................................................................................................................10 Poison Lake Chain .....................................................................................................................10 Caribou Chain .............................................................................................................................10 Other Chains and Sequences in the Caribou Volcanic Field ..............................................10 Glacial Geology ............................................................................................................................................11 Methods.........................................................................................................................................................11 Unit Names...........................................................................................................................................11 Mineral Names ....................................................................................................................................12 Reacted Phenocrysts and Mafic Inclusions ..................................................................................12 Unit Ages ..............................................................................................................................................12 Unit Labels............................................................................................................................................12 Acknowledgments .......................................................................................................................................13 Description of Map Units ...........................................................................................................................14 Glacial, sedimentary, and hydrothermal deposits .........................................................................14 Nonvolcanic sedimentary deposits ........................................................................................14 Alluvium and colluvium ....................................................................................................14 Mass-wasting deposits ...................................................................................................14 Hydrothermal deposits .....................................................................................................16 Glacial deposits .................................................................................................................16 Till ........ .......................................................................................................................16 Outwash ....................................................................................................................17 Sedimentary deposits ......................................................................................................18 Volcanic Centers of southernmost Cascades ................................................................................18 Lassen Volcanic Center ............................................................................................................18 Lassen domefield ..............................................................................................................18 Twin Lakes sequence, younger .............................................................................18 Twin Lakes sequence, older ...................................................................................24 i Eagle Peak sequence ..............................................................................................25 Bumpass sequence .................................................................................................30 Brokeoff Volcano ..............................................................................................................34 Diller sequence ........................................................................................................34 Mill Canyon sequence ............................................................................................36 Rockland caldera complex ..............................................................................................38 Maidu Volcanic Center .............................................................................................................40 Stage 3 ................................................................................................................................41 Stage 2 ................................................................................................................................42 Stage 1 ................................................................................................................................43 Dittmar Volcanic Center ............................................................................................................44 Stage 3 ................................................................................................................................44 Stage 2 ................................................................................................................................45