FLOOD02685

• County: Larimer
• Basin: South Platte
• Title: Flood Geomorphology of Arthurs Rock Gulch: Paleoflood History
• Date: 1/1/1994
• Prepared For: Larimer County
• Prepared By: Elsevier Science Publishers
• Floodplain - Doc Type: Flood Documentation Report

30

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C.F. Way/homos. R.D. Jarrett/Geomo'Pholagy 11 (/994) /5-40

c

STRATIGRAPHY ~ DESCRIPTION AND RADIOCARBON DATES

FB3

B

VERTICAL SCALE,
lNCENTDoIETERS

t:

40

Ught brown, pebbly gravel, well sorted. clast
SUPPOl1ed, composed of >90% pegmatite dasts. 10-
14cmthick.
465;; 160 yr. BP (GX"5640)
Dark brown. silty sand \0 sandy silt with
disseminated organic detritus. charcoal and wood.
Matrix~supponed. granule.. and pebble-sized clasts
comprise abolJl 10'"10 of unit. near toP. about' 40 c;m
thick.
240;; 170 yr. BP(GX.15641)

.. .. __u...____ Uncontonnfty?
~~ ~ L Orange-brown. sandy silt with minor granules and
7 ~ '7 7 pebbles. and few wOOd lragmems and charcoal,
l' - .. thickness unknown. Unit may overlie FB2.

4,320;; 225 yr. 8P (GX'l5643)

Fig. 8. Measured section and radiocarbon ages, site 1. Location of section given in Fig. Sa.

5.3, Sire 3

Site 3 covers a reach of channel about 90 m long,
and is located about 0.8 km upstream from Horsetooth
Reservoir (Fig. 2). Erosional terraces mantled by flood
boulders are inset within a low-relief, eastward-sloping
colluvial apron that mantles the underlying bedrock
(Fig. 5b). FBO boulders at Site 3 (and Site 4) are the
highest and oldest flood deposits in the lower part of
the basin. The boulders lie on an old alluvial surface
(TO, Fig, 5b) that is cut into the colluvial apron. FBO
deposits are easily differentiated from all younger
deposits by their position above the active channel and
weathering characteristics (Fig. 7; Table 2).
FE I deposits are present on the outer portion of the
TI terrace at or just above the break in slope denning
the terrace riser about 0.9 to I m below the FBO deposits
(Fig. 5b). Natural exposures through the TI terrace
show up to I m of coarse gravel and boulders on top of
fine-grained colluvial deposits and bedrock. A mature
soil with a thick Bt horizon has formed on the TI terrace
alluvium and underlying colluvial deposits. The Bt
horizon is 20 cm to more than I m thick, exhibits 75YR
3/4 colors. and contains 13-36% clay relative to one
sample of oxidized alluvium containing about 3% clay
(Table 3). Because Arthurs Rock Gulch is located in
a state park. there were liinited opportunities to exca-
vate deep trenches to obtain soil data from the C-hori-
zon. In some locations the alluvial deposits are thin and

the B-horizon extends to bedrock. Although data from
unweathered parent material is lacking the amount of
clay and color of the B-horizon of soils developed on
the TI terrace indicates that the terrace is at least of
Holocene age or older, but probably not older than
middle Pleistocene age. Soil chrono-stratigraphic stud-
ies in the Colorado Front Range indicate that surficial
deposits with soil B-horizons comparable to that
described above are probably at least 100,000 years old
(Shroba and Birkeland, 1983; Birkeland, 1984). How.
ever, the lack of data on C-horizon characteristics for
the soils developed on the Tl terrace limit age esti-
. mates.
The weathering characteristics of both FBO and FB I
deposits are similar to those recorded for erratic boul-
ders on late Pleistocene (Pinedale) moraines in the
Colo~do Rocky Mountains (Birkeland, 1973, 1984;
Madole, 1976): As discussed previously, spalling of
boulder sutfaces may occur following a forest fire, and
the degree of boulder weathering could record the time
since the last episode of spalling induced by fire.
Although FBO and FB I deposits may be correlative
with ca. 40.000 year old Pinedale age erratic boulders,
the degree of weathering on FBO and FB 1 boulders
may not indicate the time since deposition because of
the possible effects of fire. Thus. FBO and FB I deposits
could be older than 40,000 yr SP.
FE2 deposits consist of clusters of imbricategrano-
diorite and pegmatite clasts that are ] to 2.8 m above

'.

Table 3-
Field. an

Location

Tl (erraCt
Site 3

TI [t:rr.aCt
Site)

Tllerrac~
$ite4

T3[erra~.
Site 4

Colluvial s
SiteS

aSoiI borize
~unsen S,
.CU:ssthan:

the acti VI
along the
riser (Fj~
long dime
ne!. FB2 (
FBlorFI
FB3 fle
limited in
about I r.
deposits a
FB4 de
atSite3.F
channel aT
deposits c
centration:
matite anc
rarely pres
erate flood
boulder fl1

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