Arizona Geological Survey OPEN-FILE REPORT OFR 08-06

Arizona Geological Survey
m) above the active channel. Exposure of very young debris‐flow deposits (unit Y3) appears to be
dynamic, changing with significant flows due to continual exhumation and burial. Some Y3 deposits may
be reworked older deposits. A prominent Y3 snout is mapped south of East Sunrise Drive. Elongate
boulder deposits (unit B) lie in Esperero Canyon downstream of the confluence with Bird Canyon. These
resemble debris‐flow levees but are likely reworked fluvial (flood) deposits.
The focus of this mapping effort was on prehistoric debris‐flow deposits. Debris‐flow deposits from 2006
were not specifically mapped although the downstream extent of these deposits are shown with a red
hatched line on map DM‐DF‐1E. Both Esperero and Bird Canyons had debris‐flow activity in 2006. Debris
flows in Esperero Canyon were limited to the upper basin and did not extend into the mapped area
(Webb and others, 2008). The extent of 2006 debris‐flow deposits in Bird Canyon is located just
upstream of the confluence with Esperero Canyon. There was no field evidence of debris flows from
2006 downstream of the confluence, although significant amounts of water‐lain sediment were
deposited downstream. This determination of downstream extent is based on field, geomorphic
evidence. It is possible that a minimal volume of the debris flows entered Esperero Canyon from Bird
Canyon, but subsequent flooding eroded any evidence.
Ventana Canyon
Ventana Canyon (Figure 1, map DM‐DF‐1F) is a narrow bedrock canyon, with a deeply entrenched
channel and extremely coarse‐grained channel material. The drainage is narrow and choked with large
boulders and numerous debris‐flow deposits. The active channel within the canyon walls is extremely
rocky and narrow, rarely exceeding 16 ft (5 m) across. As the channel exits the canyon, it remains very
narrow and is entrenched 6‐10 ft (2‐3 m) below adjacent alluvial fan surfaces. The original active
channel that drains Ventana Canyon has been highly altered by residential and commercial
development, making identification of debris‐flow deposits very challenging. Based on local topography
and channel‐bank exposures of the underlying sediment, the Ventana Canyon alluvial fan complex was
constructed mainly by debris‐flow activity. At the canyon mouth, debris‐flow levees on the alluvial fan
splay considerably; the total width of the young debris‐flow deposits widens to 1150 ft (350 m).
Latest Pleistocene to early Holocene units YI1 and YI2 are delineated in the upper reaches of Ventana
Canyon where inset relationships can be seen. In the lower reaches of Ventana Canyon, YI units cannot
be differentiated. YI1 deposits are high‐standing debris‐flow deposits, up to 16 ft (4 m) above the active
channel and are commonly inset by younger debris‐flow deposits. Some YI1 levees exhibit lengths of 330
ft (100 m), and widths of only 16 ft (5 m). A prominent YI1 snout lies to the east side of the main
channel, immediately downstream of the canyon mouth. YI2 deposits stand 10‐13 ft (3‐4 m) above the
active channel and are adjacent to YI1 levee deposits. Overall, YI2 deposits are very similar in
appearance to YI1, however their landscape position and inset relationships are adequate to place them
as their own unit. Some YI2 levees exhibit lengths of 1000 ft (300 m), and widths of only 16 ft (5 m).
Intermediate debris‐flow deposits (unit Y1) are generally 3‐10 ft (1‐3 m) above the active channel and
tend to lie at the base of slopes, acting as channel‐confining levees. Young debris‐flow deposits (unit Y2)
are generally found above the active channel 3‐7 ft (1‐2 m) and Y3 deposits are within or adjacent to the
active channel. Very young debris‐flow deposits (unit Y3) are subjected to reworking and erosion due to
their proximal location to the active channel. The exposed extent of Y3 deposits changes with significant
flows. Some Y3 deposits may include older debris‐flow deposits.
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