Abstract ........................................................ 1
Introduction .................................................... 2
Purpose and Scope of Report .................................. 2
Quaternary Alluvium .......................................... 2
Acknowledgments .............................................. 4
Methods and Definitions ......................................... 4
Stratigraphy ................................................. 4
Clast Lithology and Roundness ................................ 5
Sedimentary Features ......................................... 7
Stratigraphy .................................................... 8
Terrace Formation and Correlation ............................ 8
South Platte River Valley .................................... 9
Cache la Poudre River Valley ................................ 15
Other Tributary Valleys ..................................... 19
Lefthand Creek ........................................... 19
Boulder Creek ............................................ 20
Ralston Creek ............................................ 20
Clear Creek .............................................. 22
Bear Creek ............................................... 24
Rocky Flats Alluvial Fan .................................... 25
Clast Lithology and Roundness .................................. 27
Provenance .................................................. 27
Dispersion Paths ............................................ 28
Roundness ................................................... 35
Clast Populations ........................................... 36
Sedimentary Features ........................................... 38
South Platte River Valley North of Denver ................... 39
Cache la Poudre River Valley ................................ 42
Tributary Valleys Nearthe Mountain Front .................... 44
Clear Creek .............................................. 44
Bear Creek ............................................... 44
Ralston Creek ............................................ 47
Rocky Flats Alluvial Fan .................................... 47
Alluvium on Drainage Divides ................................ 50
Discussion and Conclusions ..................................... 50
Alluvial Stratigraphy ....................................... 50
Provenance and Sediment Dispersal ........................... 53
Depositional Processes ...................................... 54
References Cited ............................................... 55
Appendix: Lithologic Control of Rounding in Different
Clast Populations .................................... 61
Method ...................................................... 61
Application of Method ....................................... 61
Results ..................................................... 68
Figures
1. Block diagram showing alluvial surfaces of the South
Platte River and its tributaries ............................. 3
2. Map showing location of sections through valley fill,
South Platte River and tributaries ........................... 5
3. Map showing sources of data on lithology and rounding
of gravel clasts and sedimentary features, South Platte
River and tributaries ........................................ 6
4. Sections through alluvial fill of the South Platte River
valley, Greeley to Denver ................................... 10
5. Histograms showing particle size in three gravel units,
South Platte River valley ................................... 15
6. Sections through alluvial fill of the Cache la Poudre
River valley from Fort Collins to Greeley ................... 16
7. Photograph showing differences in color, texture, and
particle size between two gravel units beneath the
flood plain of the Cache la Poudre River .................... 18
8. Histograms showing particle size in gravel units, Cache
la Poudre River valley ...................................... 19
9. Sections through alluvial fill of the valleys of Lefthand
and Boulder Creeks .......................................... 20
10.Sections through alluvial fill of the valley of Ralston
Creek near the foothills .................................... 21
11.Aerial photograph showing paleochannel in Verdos
Alluvium, Ralston Creek ..................................... 22
12.Sections through alluvial fill of the valley of Clear
Creek ....................................................... 23
13.Sections through alluvial fill of the valley of Bear
Creek ....................................................... 24
14.Map showing 3-dimensional view of Rocky Flats fan ........... 25
15.Map showing thickness of Rocky Flats Alluvium ............... 26
16.Map showing topography of base of Rocky Flats Alluvium ...... 27
17.Map showing distribution of volcanic rocks and
amphibolite + schist + diabase in alluvium, South
Platte River and tributaries ................................ 29
18.Map showing distribution of vein quartz and quartzite
in gravel alluvium, South Platte River drainage basin ....... 32
19.Photographs of outcrop of bedded Precambrian
metaquartzite in Coal Creek Canyon and metaquartzite
clasts in gravel at Riley Mound ............................. 34
20.Transect showing pebble lithology from Rocky Flats
to Beebe Draw ............................................... 35
21.Pebble roundness in alluvium of the South Platte
drainage basin .............................................. 37
22.Sketches and photograph of pit walls at the Howe
gravel pit, South Platte River, showing gravel units
and sedimentary features .................................... 39
23.Photograph of gravel units and sedimentary features,
North Dahlia pit, South Platte River valley ................. 40
24.Photographs of channel and bar features, North Dahlia
gravel pit, South Platte River valley ....................... 41
25.Photograph of sedimentary features in undivided gravel
at the Taft Hill gravel pit, Cache la Poudre River
valley ...................................................... 42
26.Photographs of sedimentary features in lower and upper
gravel units, Cache la Poudre River valley .................. 43
27.Photographs of pit face in Louviers Alluvium, Clear
Creek, showing sedimentary features ......................... 45
28.Photographs of Louviers Alluvium, Bear Creek valley,
showing sedimentary features and boulder size ............... 46
29.Photographs of Verdos Alluvium, Ralston Creek valley,
showing sedimentary features ................................ 48
30.Photographs of Rocky Flats Alluvium, Rocky Flats,
showing sedimentary features ................................ 49
31.Photographs of Rocky Flats Alluvium, Arvada, showing
sedimentary features ........................................ 51
32.Simplified sections of late Pleistocene and Holocene
terraces and fill, South Platte River, Cache la Poudre
River, Clear Creek, and Bear Creek .......................... 52
A1.Poorly rounded quartzite gravel, Verdos Alluvium,
Rocky Flats ................................................. 63
A2.Poorly rounded gravel, mostly gneiss, amphibolite,
and quartzite, Verdos Alluvium, Ralston Creek ............... 64
A3.Moderately rounded gravel, pegmatite and gneiss, some
vein quartz, and volcanic rock, Slocum Alluvium,
Bear Creek .................................................. 65
A4.Moderately rounded gravel with granite, pegmatite,
gneiss, vein quartz, and quartzite, Verdos Alluvium,
divide between South Platte River and Big Dry Creek ......... 65
A5.Well-rounded gravel, mostly granite and gneiss,
Louviers Alluvium, Clear Creek .............................. 67
A6.Well-rounded gravel, mostly gneiss, Holocene Alluvium
(upper gravel unit), South Platte River ..................... 69
Tables
1. Pebble lithology and roundness in single samples of
gravel, early Pleistocene to Holocene alluvium,
South Platte River and tributaries .......................... 30
2. Pebble lithology and roundness in composite samples
of gravel, Broadway and Holocene alluvium of the
South Platte River and tributaries .......................... 33
A1.Lithology versus roundness of pebbles in Rocky
Flats and Verdos Alluvium, Rocky Flats and upper
Big Dry Creek ............................................... 63
A2.Lithology versus roundness of pebbles in Verdos and
Piney Creek Alluvium, Ralston Creek ......................... 64
A3.Lithology versus roundness of pebbles in Rocky
Flats, Verdos, and Slocum Alluvium derived from
Green Mountain .............................................. 65
A4.Lithology versus roundness of pebbles in Rocky
Flats and Verdos Alluvium, divide between Big Dry
Creek and South Platte River ................................ 66
A5.Lithology versus roundness of pebbles in Louviers
and Broadway Alluvium, Clear Creek .......................... 67
A6.Lithology versus roundness of pebbles in Broadway
and Holocene Alluvium, South Platte River north of Denver ... 69
A7.Lithology versus roundness of pebbles in Broadway
and Holocene Alluvium, Cache la Poudre River ................ 70
A8.Summary of correlation of rounding with lithology,
as predicted by chi-square tests ............................ 70
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