Preliminary report on deposit models for sand and gravel in the Cache la Poudre River valley

The stratigraphy, sedimentary features, and physical characteristics of gravel deposits in the Cache la Poudre River
valley were studied to establish geologic models for these deposits. Because most of the gravel mined in the valley is
beneath the low terraces and floodplain, the quality of these deposits for aggregate was studied in detail at eight sites in a
25.5-mile reach between Fort Collins and Greeley, Colorado. Aggregate quality was determined by field and laboratory
measurements on samples collected under a consistent sampling plan.
The Broadway terrace is underlain by Pleistocene alluvium and, at some places, by fine-grained wind-blown
deposits. The Piney Creek terrace, low terraces, and floodplain are primarily underlain by Holocene alluvium.
Pleistocene alluvium may underlie these terraces at isolated locations along the river. Gravels beneath the Piney Creek
terrace, low terraces, and floodplain are divisible into two units that are poorly distinguishable at the upstream end of the
study area, but are readily distinguishable about 7 miles downstream. Where distinguished, the two gravel units are
separated by a sharp, locally erosional, contact. The upper gravel is probably of Holocene age, but the lower gravel is
considered to be Holocene and Pleistocene.
The primary variation in particle size of the gravels beneath the floodplain and low terraces of the Cache la Poudre
River valley is the downstream decrease in the proportion of particles measuring 3/4 inch and larger. Above Fort Collins,
about 60 pct of the gravel collects on the 3/4 inch sieve, whereas about 50 pct of gravel collects on the same sieve size at
Greeley. For 1.5-inch sieves, the corresponding values are about 50 pct for Fort Collins and only about 30 pct for
Greeley. Local differences in particle size and sorting between the upper and lower gravel units were observed in the
field, but only the coarsest particle sizes appear to have been concentrated in the lower unit.
Field measurements of aggregate quality, pebble lithology, and shape show little significant downstream variation.
Pebble lithology is about 25 percent granite; 48 percent pegmatite; 5-7 percent each of gneiss, quartz, and quartzite; and
minor amounts of diabase, schist, volcanic porphyry, and sandstone. Among the rock types, only the volcanic porphyries
might be reactive with Portland cement.
Pebble shape is dominantly equidimensional with a tendency to form thick, disc-shaped particles. Disc-shaped and
spherical particles comprise about 39 percent and 31 percent of the pebble-size fraction, respectively. Rod and blade
shapes comprise about 18 and 12 percent of the pebble-size fraction, respectively. The relatively large proportion of
equidimensional particles in the Cache la Poudre may be due to the small proportion of layered gneiss in gravel. Pebbles
having axial ratios less than 0.5, which might be structurally weak, are rare.
The two gravel units show subtle local differences and evidence for derivation of the younger gravel from the older
gravel. At many sites, the upper gravel unit tends to contain more quartz plus quartzite, has poorer physical quality, and
contains more angular pebbles than the lower gravel. Weathering, followed by transport in the river, might be expected to
concentrate quartz and quartzite, degrade physical quality, and break pebbles into angular fragments. This conclusion is
consistent with local evidence of an erosional contact between the two gravel units.