Arizona-Nevada Academy of Science

The Geology of Two Small Cenozoic Volcanoes in Southwestern Arizona Author(s): Shelby Cave and Ronald Greeley Reviewed work(s): Source: Journal of the Arizona-Nevada Academy of Science, Vol. 37, No. 2 (2004), pp. 105-110 Published by: Arizona-Nevada Academy of Science Stable URL: http://www.jstor.org/stable/27641729 . Accessed: 27/11/2011 20:48
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TheGeology of Two Small Cenozoic Volcanoes in Southwestern Arizona
Shelby Cave andRonald Greeley, Department of Geological PO Box 871404, Tempe, AZ 85287-1404 Sciences, Arizona State University,

Abstract to Early in the late Pliocene shield volcanoes and Gillespie erupted near the Gila River Arlington are located 60 and 70 km west of Phoenix, AZ, respectively. Each shield consists of 3-4 alkali and Pleistocene olivine basalt units emplaced in three eruptive phases of decreasing volume. The lava flows of the initial phase km south. At the channel was shifted -0.5-1 into the river channel. At Arlington of each shield flowed the channel was dammed by lavas and possibly formed a lake. The river cut a new channel located Gillespie ismantled by alluvium, possibly due to a change in the ~1.5 km to the east. The northern margin of Arlington alluvial sediments deposited to the formation of the paleolake. Although Hassay ampa Riv-er base level related There is no evidence of weathering carbonate accumulation. and pedogenic incision by ephemeral washes, and that some of the between basalt units. This suggests that the eruptions were essentially monogenetic are inaccurate. and 4.3-1.35 Ma for Gillespie dates that span from 3.28-1.28 Ma for Arlington radiogenic

on Arlington probably indicate that it is older thanGillespie, both shields exhibit comparableweathering,

are two small (~5 km and Gillespie Arlington located in south diameter) basaltic shield volcanoes western Arizona ~60 km west and 70 km southwest and of Phoenix, (Fig. 1). Arlington respectively consists of multiple flows of alkali olivine Gillespie of volcanoes basalt and are the two northeasterly from the Sentinel four small shields that trend N45 ?E

Introduction

at slope initiation points, distributaries including breaks, and tributary patters that appear to be con scarps that could patterns. vegetation as small be flow fronts;

trolled by geology; (3) changes in topography such and (5)

Plains lavafield (Fig. 1).The Sentinel Plains and the shields are of similar age and composition (Leeman 1970, Shafiqullah et al. 1980, Lynch 1989). Arling at small scales by ton and Gillespie were mapped

Reynolds and Scotnicki (1993), Demsey (1989), were Wilson et al. (1957), and Reynolds (1988), and dated at 3.28-1.28 Ma and 4.3-1.35 Ma, et al. 1986). The dates were acquired in Reynolds that included conjunction with hazard assessments surveys of the Gila River terraces by Lee and Bell

(Eberly and Stanley 1978, Shafiqullah et al. 1980,

respectively

Gillespie shield dammed (1975),who suggested that and diverted the Gila River. The Gila River has in its current course for ~5 Ma (Smith 1970).

flowed

Approach
A

survey of previous maps, well log data, and followed by analysis publications was completed, and field mapping. The data collected were synthe sized to derive two geologic maps (Figs. 2, 3) and geologic histories. The basaltic units were defined such as: (1) lava flow textures using characteristics debris the nature of the in situ weathered including on the surface of the flow and various surface pat terns; (2) superposed patterns in ephemeral washes

Figure 1. The Sentinel-Arlington Volcanic Field (SA VF) consists of Pliocene toPleistocene alkali olivine basaltic eruptive centers, including the Sentinel Plains lavafield and four small shields trending ~N45 E across the Gila Bend Mountains, including Warf ord Ranch, Woolsey, and Arlington. Gillespie

Cave, S., and R. Greeley. 2004. The geology of two small Cenozoic volcanos insouthwestern Arizona. Journal ?2004 Shelby Cave and Ronald Greeley. of the Arizona-NevadaAcademyof Science 37(2):105-110.

106

Geology

of two small Cenozoic

volcanos

4- Cave and Greeley

I

Summit Region \ LEGEND m dike ^4-5-7 ? ? 3-5 m dik? -.1-3mdik?

? 33'22* 30*

\Will

*.

I? W2ff

Flow Festoons Summit Dikes
Figure 2. Geologic map of Arlington small shield volcano.

Gila River

Geology of two small Cenozoic volcanos + Cave andGreeley

107

is altered to dark reddishbrown to dark purplish red where it is in contact with PAb2, as seen in cross section exposures. Well-log data on the southeastern and southwestern margin of the flow record river indicate a channel deposit that gravels that possibly extends beneath the southern margin of RAM. EAb2 is characterized by moderate slopes (-2?) and faint flow festoons spaced -25-100 m apart visible in the aerial photograph represent (Fig. 3). The festoons transverse flow buckling of the cooling surface as to flow. More flow festoons the interior continued are exposed on the western side of the edifice than on the eastern side, although the size and spacing of festoons are similar. PAb3 is a third basaltic unit and is marked by slopes of 6.5?-17?, minor scarps at which several ephemeral washes are initiated, and a lack of flow festoons. PAb4 is characterized by a surface texture and minor scarps cor homogenous to initiation of 80% of the ephemeral responding PAb4 might washes. pyro represent a weathered clastic deposit. shield has two peaks -0.25 km apart. Arlington The northwestern peak is 357 m in elevation and has cone dikes approximately three discontinuous 35 m,

_?_???___________________^_______________?___?__?

^

/ fL \

0

200

i

direction

Figure 3. Aerial photograph and corresponding diagram of part of the west side of Arlington shield, showing flow festoons used to distinguish unit PAb2 from other units. Flow festoons form during final stages offlow emplacement; festoons of this size are typical of a'aflows, in contrast to the smaller corded features found on pahoehoe basalts.

-1 to 7 m thick and stand as high as 2 m above the surface. The southern and eastern dikes dip toward the interior at 60?-90?, whereas the northern and western dikes dip toward the interior at 45?-60?. The suggests that the NW peak edifice was asymmetry tilted after the dikes were emplaced to the northwest, summit is away from the SE peak. The southeastern -310 m elevation and forms a plateau -75 m in diameter with no scoria deposits or other evidence for vent activity. The surface of Arlington includes a mixture of pedogenic calcium carbonate rubble, basaltic clasts, and silt to clay-sized fines. The basaltic clasts are dioxide heavily varnished with a rind of manganese as thick as 2 mm and often exhibit oxidation or ped ogenic calcium carbonate coatings on their under carbonate and/or clay within side, and calcium cracks. The clays and silts are cream-colored and are to be primarily windblown considered deposits, although some could be derived from in situ weath ering. Hassayampa River deposits cover the northern margin of RAM. The alluvium consists of silt, sand,

50m, and 150m in diameter (Fig. 2). The dikes are

and rounded gravel (up to 2 cm in diameter)with scattered channel lenses -1 to 2 m across by 0.5 m thick consisting of subangular vesicular basaltic

Results and Discussion
Arlington shield is composed

gravels. of four flow units

Gillespie shield is composed of flow units PGb 1,
PGb2, PGb3 and summit dikes partially surrounded edifice by a spatter rampart (Fig. 4). The Gillespie two pre-emption volcano embays mid-Tertiary clastic buttes and encircles a low granitoid kipuka.

(Fig. 2; PAbl, PAb2, PAb3, PAb4) and several cone dikes in the summit area. PAbl is a dark gray basalt lava flow characterized by low surface slopes (