U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY An introduction to sand and gravel deposit models, Front Range Urban Corridor by David A. Lindsey1 U.S. Geological Survey Open-File Report 97-81 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 1Lakewood, Colo. 80225 U. S. GEOLOGICAL SURVEY OPEN-FILE REPORT AN INTRODUCTION TO SAND AND GRAVEL DEPOSIT MODELS, FRONT RANGE URBAN CORRIDOR by David A. Lindsey DEFINITION quality or suitability, such as particle size and Sand and gravel deposit models are composition. Deposit size (3) is measured in descriptions of deposits that may be mined for areal extent, thickness, and volume of gravel in natural aggregate. Commonly, a mineral deposit entire geologic units such as alluvial terraces model consists of a systematic arrangement of along streams (Bliss, 1993). Environmental information that summarizes the features of a effects of mining (4) may be described in terms group of similar deposits (Cox and others, that take into account the size of the area 1986). Deposits in each group are thought to affected, mining methods, overburden, depth to have formed in more or less the same manner water table, and production of fine-grained and to share similar features. The information in waste. a model is descriptive, but judging the significance of information may depend on an EXAMPLES OF MODELS understanding of how the deposit formed. By Models of sand and gravel deposits in the comparing features specified by mineral deposit Front Range urban corridor are named for their models with the characteristics of an area of physiographic setting or mode of formation interest, models can be used to explore for (Figure 1). The models represent a variety of mineral deposits and assess potential for geologic environments that combine a history of undiscovered deposits. erosion by streams, deposition of sand and Deposit models commonly contain gravel and, commonly, renewed erosion. The summaries of information about 1) geologic resulting deposits have characteristic landforms environment, 2) deposit characteristics, 3) such as dissected alluvial fans and river terraces deposit size, and 4) possible environmental at various levels. In the case of eolian sand, the effects of mining. For sand and gravel deposits, model is characterized by sand and silt the geologic environment (1) refers to the deposited as dunes on upland areas during present geologic setting, which is the product of periods of aridity; at present, the dunes have both the sedimentary environment in which the been stabilized by vegetation. deposit formed (Langer, 1988; Smith and Collis A working classification of sand and gravel 1993, p. 16-30) and the subsequent geologic deposit models (Table 1) of the Front Range history of the site of the deposit. Deposit urban corridor was compiled from previous characteristics (2) include information on research. Researchers for the Front Range physical properties such as sedimentary features Infrastructure Resource Project will revise the and information that is relevant to aggregate classification and description of models as more Figure 1.--Block diagram showing geologic environments and landforms associated with sand and gravel deposits of the Front Range urban corridor, Colorado (from Schwochow and others, 1974a). A, alluvial fan; T1-T3, high terraces; F, low terrace; E, eolian sand. Table 1.—Sand and gravel deposit models, Front Range Urban Corridor, Colorado (compiled from Schwochow and others, 1974a;b; Colton and Fitch, 1974; and Trimble and Fitch, 1974a;b) MODEL NAME GEOLOGIC DEPOSIT DEPOSIT ENVIRONMENTAL ENVIRONMENT CHARACTERISTICS SIZE EFFECTS Dissected fan Upland alluvial Massive debris- and Small to ? fan, now an stream-flow deposits; large interfluvial low quality: poorly surface sorted, weathered High dissected Eroded alluvial Channel gravel; low Small Small excavations terrace fill; now an quality: well-sorted, interfluvial weathered surface High terrace Alluvial fill; Channel gravel; high Large Large excavations surface above quality: graded stream level sorting, weakly weathered Low terrace Alluvial fill; Channel gravel; high Large Large excavations at surface near quality: graded stream level stream level sorting, unweathered Eolian sand Upland dune Sand and silt; sand Variable Possible erosion of field; stabilized suitable for specialty disturbed area by vegetation uses; uniform particle size complete information is collected on deposit information on deposits as well as from direct characteristics, size, and possible environmental observation of mining operations. effects of mining. DATA FOR MODELS PREPARATION OF MODELS Deposit maps (Figure 2) show the areal The working classification of sand and gravel extent of various gravel deposits; maps can also deposit models for the Front Range Urban show the thickness, depth, and depth to water Corridor is based on preliminary analysis of three table based on drilling or geophysical studies kinds of available data: 1) deposit maps showing (Dunn, 1991; Smith and Collis, 1993, p. 4168). distribution of sand- and gravelbearing units, 2) These features are commonly shown on contour subsurface sections, based on borehole logs maps (e.g., Robson, 1996). Aggregate quality, showing depth and thickness of sand, gravel and based on physical, chemical, and mechanical other sediment, and 3) compilations of particle properties (Marek, 1991), can also be shown on size and composition, which are useful for maps. Many maps of aggregate resources, such assessing aggregate quality or suitability. From as those of the Front Range Urban Corridor these data, the geologic environment can be (Colton and Fitch, 1974; Trimble and Fitch, identified and deposits classified into the 1974a;b), are derived from geologic maps appropriate model for description. Deposit (Colton, 1978; Trimble and Machette, 1979a;b). characteristics can be identified by integrating Digital geologic maps (e.g., Green, 1992) are data on particle size and composition with maps especially suitable for rapid reformatting and and subsurface sections. Statistical models of interpretation of sand and gravel resources. deposit size can be prepared from dimensions of Map units can be recombined in a variety of individual deposits shown on maps and cross- ways to show the distribution of resources by sections. Environmental effects of mining can quality, suitability, deposit model, or other be assessed from available maps and criteria. The deposit map shown in figure 2 104°52'30" 104°45' 40° EXPLANATION EOLIAN SAND BRIGHTON LOW TERRACE HIGH TERRACE 85 DISSECTED G R A V E L S E L V A G R PEDIMENT 20 ft 40 ft 20 ft 80 ft 40 ft 20 ft 76 UNCONSOLIDATED 60 ft SEDIMENTS, THICKNESS - IN FEET - - - 40 ft - ABANDONED CHANNEL - 40 ft- - - 20 ft - 0 1 MI 39°52'30" Deposits from Schwochow and others (1974); thickness of unconsolidated sediments from Robson (1996) Figure 2.2.—Map showing sand and gravel deposits of the Brighton area, Colo., classified by model. 5100 B1-67-26ccc B1-66-33bba 5050 B1-66-29dcd 5000 B1-67-25dcd2 B1-66-30dcd B1-66-30ddd2 B1-66-29cdc South Platte Base of low River High terrace terrace fill (?) Upland deposits Low terrace 4950 Vertical Exaggeration ? ? LEVEL ABOVE SEA FEET 40:1 4900 EXPLANATION DRILL GRAVEL > SAND SAND > GRAVEL SILT AND CLAY SOIL HOLE Figure 3.3.—Subsurface section, South Platte River Valley, Brighton, Colo., showing gravel layers. Table 2.—Definitions of particle size used in the Front Range Urban Corridor UNIT OF SILT AND SAND GRANULE PEBBLE OVERSIZE MEASURE CLAY GRAVEL GRAVEL Size in 0.0025 0.0025-0.079 0.079-0.158 0.158-2.5 >2.5 inches Size in mm <0.0625 0.0625-2 2-4 4-64 >64 U.S. Sieve <230 230-10 10-5 5-0.4 <0.4 Series in mesh/inch classifies resources by sand and gravel deposit provided by Pettijohn (1975, p. 52-64). Shape model. is defined by the ratios of the three dimensions Subsurface sections (Figure 3), based on of a particle: particles tend to be spherical, rod- drillhole logs and geophysical exploration, show shaped, disc-shaped, or blade-shaped. Thin, the thickness, depth, and lateral extent of sand blade-shaped particles tend to be weak, whereas and gravel along a line. Thickness of overburden equidimensional particles tend to be strong. and interbedded material that is too fine for use Roundness refers to the degree to which a as aggregate can also be shown. The section particle lacks angular corners; the greater the shown in figure 3 was prepared from drillhole roundness, the weaker the bond with a logs reported in the literature (Schneider, cementing agent. The degree of roundness is 1962). Uniform and accurate description by a determined by visual comparison with particle trained observer is essential if drillhole logs are images assigned to a roundness scale ranging to be used to construct accurate subsurface from very angular to well-rounded. A variety of sections. Geophysical exploration is usually surface textures and coatings also affect the calibrated with drillhole logs. Subsurface bond between a particle and its cement. No sections are particularly useful for three- data on particle shape, roundness, and surface dimensional visualization of a deposit. textures in sand and gravel deposits of the Front Particle size, shape, and composition are Range Urban Corridor have been identified. among the properties that affect the suitability Composition refers to rock types, contained of an aggregate deposit for a specific minerals, and chemical constituents, all of which commercial use (Marek, 1991; Smith and Collis, are important to determining aggregate quality. 1993, p. 145-306; Langer and Knepper, 1995). Certain rocks contain silica minerals that react Maximum particle size is important because it with Portland cement, resulting in rapid determines requirements for crushing deterioration of concrete.