BRUCE M. CROWE \ Department of Geological Sciences, University of California, Santa Barbara, RICHARD V. FISHER j Santa Barbara, California 93106 Sedimentary Structures in Base-Surge Deposits with Special Reference to Cross-Bedding, Ubehebe Craters, Death Valley, California Note: This paper is dedicated to Aaron and Elizabeth more km2. The volcanic field is named from Waters on the occasion of Dr. Waters' retirement. the largest crater, Ubehebe. Following the recognition of base-surge depositions in the rim beds of Ubehebe Crater ABSTRACT (Fisher and Waters, 1969, 1970), the present Ubehebe craters, Death Valley, California, study was undertaken to evaluate in greater include over a dozen maar volcanoes formed detail the physical characteristics of base-surge primarily by phreatic eruptions of trachybasalt deposits in order to gain possible insights into through a thick and permeable fanglomeratic flow mechanisms of base surges. Particular sequence on the north slope of Tin Mountain. attention is given here to the bed forms de- Tuff derived from Ubehebe Crater, the scribed as antidunes at Ubehebe by Fisher and largest crater in the area, is characteristically Waters (1970). thinly bedded or laminated and was deposited The Ubehebe craters originated on the by airfall and base-surge processes. Thick- gullied northern slope of Tin Mountain in late bedded deposits showing evidence of mass flow Pleistocene or Holocene time following the dis- occur where base surges were concentrated appearance of ancient Pleistocene (?) lake within, and followed gullies which had been waters. About 4 km north of the craters, tuff carved into the fanglomerate prior to eruption. from Ubehebe rests on lake deposits exposed Cross-bedded sequences were deposited by in the valley floor. The ancient lake has been base surges that moved radially outward from named Lake Rogers and was present during Ubehebe Crater. They occur in the form of Pleistocene time (Clements, 1954). The vol- relatively small and large dunelike structures canoes exploded through, and their deposits with spacing and morphologic features similar rest upon, a fanglomeratic sequence at least 500 to antidunes and migration patterns somewhat m thick. similar to climbing ripples. The largest dunes in the area are composite structures that preserve X a sequence of bed forms deposited in the high flow regime. Deposition apparently began in "N X'« \ \ the antidune phase of the upper flow regime, \ Ubthibt progressing in time through sinuous lamination i to plane beds as flow power decreased. Lamina- t tions are well developed and bed forms are preserved at each level within the composite \ California structures because of a high rate of deposition Ubehebe and high sediment cohesion during flow of the Craters base surges. V-, ^f ^ %\ INTRODUCTION £ Tin % 1 km IN \ J Mountain • ' The Ubehebe craters area, located in Death See it Valley, California (Fig. 1), includes 13, possibly 16, volcanic craters within an area of about Figure 1. Index map showing location of Ubehebe 3 km2. Ejecta from the craters covers 15 or craters area. Geological Society of America Bulletin, v. 84, p. 663-682, 14 figs., February 1973 663 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/2/663/3429009/i0016-7606-84-2-663.pdf by guest on 27 September 2021 664 CROW2 AND FISHER The fanglomerate consists of thick-bedded to The largest crater, Ubehebe Crater, is a massive, highly lenticular beds of sandstone roughly circular tuff ring (terminology of and conglomerate with pebble clasts composed Fisher and Waters, 1970) 0.7 to 0.8 km in predominantly of volcanic and metamorphic diameter and 235 m deep. The diameter of the rock varieties with lesser amounts of sedimen- crater at the original eruptive surface (top of tary and plutonic fragments. The sandstone fanglomerate) is about 0.6 km. The shape of interbeds and matrix of the coarse-grained the crater is modified in the northwestern and layers are composed of lithic fragments and southeastern parts by preemption topography minerals derived from materials as diverse as and posteruption slumping. the large clasts. The rocks are poorly sorted, Ejecta from Ubehebe Crater attain a maxi- weakly cemented with calcite, and highly mum thickness of 50 m at the crater rim. The permeable. deposits generally decrease in thickness radially The Ubehebe craters are here divided into from the crater and dip gently outward at 10° (1) Ubehebe Crater, (2) Little Hebe Crater, to 15° except for local thickening and draping (3) a western crater cluster, and (4) a southern within preexisting gullies carved into the crater cluster (Fig. 2). fanglomerate. At one place on the southeast PROBABLE SEQUENCE OF ERUPTIVE EVENTS OF THE UBEHEBE CRATERS Nonphreatic vulcanian eruptions developed cinder cone located near crater no. 1; final stages of activity Strombolian. Vent area of cinder cone may have coincided with vent area of crater no. 1. Phreatic eruptions formed crater no. 1 and destroyed cinder cone. Crater no. 2 developed approxi- mately contemporaneously with crater no. 1—both are explosion craters with little rim ejecta. Cra- ter nos. 3 and 4 developed after crater no. 1 and prior to Ubehebe Crater. Phreatic explosions of Western Crater Cluster may have been contsmporaneous with activity from Slouthern Crater Cluster. Mild volcanic activity from Little Hebe Cra:er developed small spatter cone which was disrupted by phreatic base-surge forming eruptions from same vent, greatly widening Little Hebe Crater. Repeated, phreatic, base-surge forming eruptions formed Ubehebe Crater; ejecta from Ubehebe Crater covers all craters in volcanic field. Figure 2. Index map showing crater designation and probable sequence of eruptive events. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/2/663/3429009/i0016-7606-84-2-663.pdf by guest on 27 September 2021 BASE-SURGE DEPOSITS, UBEHEBE CRATERS, CALIFORNIA 665 side of the rim, inward dipping strata are rim sequence include agglomerate (on the preserved. south side) derived from early cinder cone Inside the eastern portion of the crater below activity and a thin sequence of tuff identical the rim deposits are excellent exposures of the in kind to beds from Ubehebe which were basement fanglomerate. It is intensely fractured derived from earlier phreatic eruptions within and faulted, probably due to explosive crater the southern and western crater cluster groups. formation rather than tectonic processes. It is estimated that well over three-fourths of The western half of Ubehebe Crater has the 50-m rim-bed tuff sequence originated from been modified by posteruption slumping, which Ubehebe. is particularly pronounced on the northwest Beds derived from Ubehebe Crater contain side where slumped debris partially covers a accidental material derived from the under- resistant white lens of volcaniclastic sandstone lying fanglomerate, accessory ejecta from within the fanglomerate. Small debris flows pre-Ubehebe Crater vulcanian activity, and which develop from infrequent rains on poorly juvenile sideromelane fragments with some consolidated tuff are active on the inside slopes bombs derived from the Ubehebe eruption. It of the crater. The relatively flat floor oc- is likely that all three kinds of ejecta from the casionally contains ephemeral lakes. earlier eruptions were reincorporated with the Little Hebe Crater is a small cone with a early ejecta of Ubehebe. Color variations of the diameter of 100 m and a depth of 20 m. It is beds reflect the abundance of the various composed of agglutinated spatter overlain by a constituents: those rich in accidental debris 5- to 8-m veneer of pale red to orange base- are grayish-brown, those with abundant basaltic surge deposits derived from Little Hebe, and fragments are dark gray, and beds rich in is draped with gray tuff beds derived from sideromelane are light gray. Ubehebe Crater. Under the microscope, juvenile ejecta are The southern crater cluster consists of four hyalopilitic, although some samples are pilo- probable craters greatly modified by erosion, taxitic due to microlite orientation during by the breaching of crater walls by adjacent aerial flight. Phenocrysts include kaersutite, craters (including Ubehebe), and by partial augite, and plagioclase. Plagioclase microlites covering with volcanic ejecta from Ubehebe are andesine to calcic-andesine; micropheno- Crater. Little Hebe Crater is nestled in the crysts and phenocrysts (calcic-oligoclase to northernmost crater (crater no. 1) of this andesine) are highly resorbed with cloudy cores cluster. filled with inclusions of glass, iron oxides, and The western crater cluster consists of seven rare clinopyroxene; thin sodic rims surrounding craters (including two craters somewhat south the cloudy cores are common. Kaersutite of the aligned crater cluster). The craters are occurs as scattered phenocrysts and micro- highly eroded and are draped with ejecta from phenocrysts but does not occur in the ground- Ubehebe Crater. Most are explosion craters mass. It is yellow-brown in plane light, has a with little rim ejecta, although the larger ones 2VX of about 67°, and is invariably surrounded appear to have sufficient ejecta surrounding by opacitic rims of magnetite and clino- their vents to be considered tuff rings. pyroxene. Clinopyroxene, occurring as pheno- Ejecta from Ubehebe Crater are most crysts, microphenocrysts, and microlites, has a voluminous and cover all preexisting craters; 2VZ of 44°, indicating that it