ERUPTIVE STRATIGRAPHY OF THE UBEHEBE CRATER CLUSTER, DEATH VALLEY, CA

Eruptions from the Ubehebe Crater cluster in Death Valley were short-lived, emplacing a sequence of phreatic, phreatomagmatic, and magmatic deposits from ~13 vents in Holocene time. Seven craters form the main group, which erupted over 1.5 km, north-south. At least five more make a 500-m east-west alignment west of the main crater group. One more is an isolated shallow crater ~400 meters south of that alignment. All erupted through Miocene fanglomerate and sandstone, which are now distributed as comminuted matrix and coherent lithic clasts in all Ubehebe deposits. Stratigraphic evidence showing that all Ubehebe strata erupted within a short time interval includes: (1) Deposits from the many Ubehebe vents make a multi-package sequence that conformably drapes paleo-basement topography with no erosive gullying between emplacement units; (2) Several crater rims formed early in the eruptive sequence are draped by subsequent deposits; and (3) Tack-welded to agglutinated spatter and bombs that erupted part-way through the sequence remained hot enough to oxidize the youngest emplacement package. In addition, all deposits consolidated enough to drill yield reliable paleomagnetic directions, with site mean directions showing no evidence of geomagnetic secular variation. Chemical analyses of juvenile components from every eruptive package yield a strikingly narrow compositional range in major elements [SiO₂ (49.01-50.11); MgO (4.98-5.35); K₂O (2.27-2.39)] and trace elements [Rb (28-32); Sr (1531-1588); Zr (373-404)]. Despite lithologic similarities, individual fall units can be traced outward from vent by recording layer thicknesses, maximum scoria and lithic sizes, and juvenile clast textural variations. This permits a reconstruction of the eruptive sequence, which shows changing eruptive styles (including phreatic, phreatomagmatic and magmatic) throughout. The largest and northernmost of the craters, Ubehebe Crater, is the youngest of the group. Its largely phreatomagmatic deposits drape all of the others, thicken in paleogullies and thin over the other newly created crater rims. Evidence in-hand virtually requires that the Ubehebe cluster of craters erupted over a brief time interval, not protracted over centuries.