Understanding Colorado Geology; A Powerful Tool for

Site Evaluation

Presented By: Matthew Hopkins

CTL Thompson Breckenridge/CPOW

Three Parts

• Geology 101 (Rocks for Jocks)

• Brief History of Colorado

• How to apply this Knowledge to Site Evaluation

Geology 101• Three Types of Rocks

1. Igneous – From the Latin word for “fire”

• Made of solidified molten material (magma)

• Crystals are well defined and interlocking (crystalline)

2. Sedimentary – From sediment

• Laid down in layers

• Many depositional environments

• Preserves clues as to what the earth was like at the time of deposition

3. Metamorphic - Meta = change, Morph = form

• Altered preexisting rocks from heat and pressure

• Minerals heat and partially melt to for new and different minerals

Intrusive Igneous

• Also known as plutons or intrusions

• Can form large “batholiths” – Pikes Peak granite is over 1,300 square miles

• Can form smaller dikes and sills that force through existing rock.

• Examples: granite, gabbro, pegmatite, diorite

Extrusive Igneous

• Cooled above ground rapidly

• From volcanic eruptions of lava

• Basalt, rhyolite, and obsidian

• Tuff and welded tuff is from consolidated volcanic ash

• expansive clays

Metamorphic

• From heat and pressure

• Contact Metamorphism

• Regional Metamorphism

• Preexisting rocks partially melt to form new minerals

• Types of minerals indicate the degree of metamorphism

• Some of the oldest rocks in the word

• (Black Canyon rocks 2 byo)

• Gneiss, schist, quartzite

Sedimentary• Sandstone – made up of sand

sized partials

• Shale and mudstone – made up of clay sized particles

• Limestone and dolomite –made up of calcium carbonate usually from shells and exoskeletons of sea creatures (coral reefs)

• Evaporites – minerals in solution precipitate out during the evaporation of an inland sea/playa lake (gypsum, halite)

• Generally younger rocks

Common Depositional Environments in CO

• High energy = large particles; Low energy = small particles

• Alluvial: rivers and streams; all grain sizes, well sorted

• Alluvial fans/debris fans; poorly sorted clayey sand to boulders

• Lacustrine: lakes; silt and clay, well sorted

• Aeolian: wind; silt, clay and sand (San Luis Valley), well sorted

• Glacial• Outwash: similar to braided alluvium,

moderately to well sorted• Till: plowed material, poorly sorted, moraines

Common Depositional Environments in CO

• Colluvium: gravity transport of soil, poorly sorted (nature’s fill)

• Evaporites: Stratified minerals left after evaporation

• Gypsum, salts, anhydrite (you got me)

• Decomposed Crystalline Bedrock

• Structure of bedrock, but can excavate to sand/gravel

Colorado Geological History-300 Million Years in 15 minutes

• Mountains, erosion, deposition, mountains, erosion…

• Paleozoic Era

• Pennsylvanian Period (318-300 mya)• Colorado was at the equator• Covered by a shallow sea• Ancestral Rockies began to grow into two main

ranges: Ancestral Front Range (east) and Ancestral Uncompahgre (west)• Sediments of the Fountain formation deposited

(Flatirons, Red Rocks, Garden of the Gods, Vail cliffs)

• Rising mountains and isolated seas • Evaporite deposits of the Eagle Valley (salts,

gypsum)Credit: Ray Troll

Colorado Geological History-300 Million Years in 15 minutes

• Permian Period (300-250 mya)

• Ancestral Rockies eroded to near flat

• CO north of equator. Humid changed to arid

• Sea level dropped and sand dunes dominated the surface

• Ended with mass extinction

• Mesozoic Era-Age of the Dinosaurs

• Triassic Period (250-200 mya)

• Sahara like conditions continued

• Red and pink shales and sandstones were deposited on alluvial plains and limestone in shallow seas

• Few fossils

Colorado Geological History-300 Million Years in 15 minutes

• Jurassic Period (145-200 mya)

• CO continued to drift north

• Dunes and left thick sandstone beds in the west (Navajo Sandstone)

• Transgressions and regressions-lowland swamps

• At this time the Morrison formation deposited

• Cretaceous Period (146-66 mya)

• Transgression flooded the swamps and lowlands of CO

• Beach sands transgressed inland (Dakota Formation)

Colorado Geological History-300 Million Years in 15 minutes

• Transgression = deep seas deposited Benton Shale, Niobrara Limestone, and Pierre Shale

• Sandstone and coal beds deposited in the west• Regression again: Fox Hills and Laramie sandstone represent

the receding beaches• Laramide Orogeny begins - seas depart CO for the last time. • Ends with a bang - K/T boundary (south table mountain)

• Cenozoic Era – Mammals Rule

• Tertiary Period (66-3 mya)• Rocking time in the Rockies – mountain building from New

Mexico to Alaska• Early Tertiary - Front range uplifted pushing Precambrian

metamorphic rocks up in the center.• Sedimentary rocks tilted vertically along the eastern front

(Garden of the Gods, Flatirons, hogbacks.

Colorado Geological History-300 Million Years in 15 minutes

• Deep sediments deposited on the eastern plains. (Denver Formation, Dawson Arkose, Castle Rock Conglomerate)

• Vulcanism abundant throughout the mountains

• San Juan Mountains, Rocky Mountain National Park, Granby area, Rabbit Ears Range all saw extensive volcanic activity

• Late Tertiary characterized by continental spreading and rifting

• Down drop blocks and uplifted mountains

• Great basins such San Lois Valley, Rio Grand Basin, Blue River Valley, Upper Arkansas Valley were pulled apart.

• Vulcanism once again flooded the valleys with basalt in areas along with thick sediments.

• Adjacent mountains such as Sangre de Cristo, Gore, Williams Fork, and Tenmile Ranges were thrust up

Colorado Geological History-300 Million Years in 15 minutes

• Quaternary – Ice Ages (3 mya – Now)• Colorado now at a mid-northern latitude of today

• Alpine glaciers formed primarily above 8,000 feet in multiple advances (Bull Lake and Pinedale, etc.)

• Carved U-shaped valleys and cirque lakes. Rivers with melt water carved deep gorges (Royal Gorge, Black Canyon)

• Moraines, till, and outwash was deposited

• Sediment laden rivers covered the plains (South Plate)

• Dunes and loess deposited from receding glaciers (Great Sand Dunes NP)

Physiographic Provinces of CO• Great Plains

• Gently dipping sedimentary rocks to the east as thick as 10,000’

• Recent sediments

• Mountains • Intrusive/extrusive igneous• Old metamorphic rocks at the

core – overlying sedimentary rocks eroded away

• Steeply dipping, folded, faulted sedimentary on the flaks of ranges

• Glacial deposits, alluvium, colluvium

• Colorado Plateau• Flat to gently dipping

sedimentary and evaporites• Localized intrusive/extrusive

igneous Credit: Colorado Geological Survey

Applying Geology to Site Assessment

• Know before you go

• Common Resources • Web Soil Survey – works well in some places and not in others (plains?)

• CPOW shows this well

• NWI Mapper - https://www.fws.gov/wetlands/data/mapper.html• Wetlands and water bodies requiring setbacks

• Google Earth

• Geological Maps: https://ngmdb.usgs.gov/mapview/• Unsung resource

• Inconsistent quality but a valuable resource

Geologic Maps• Strat column

• Shows the geologic units on the map

• Youngest to oldest

• First letter indicates the age (usually)

• Map

• Shows the location of the units at the surface

• Strike and dip

• Faults

• Cross section (B-B’)

• Fault displacement

• Structure/dip/orientation of strata

• Description of Map Units

• Some maps have a separate narrative packet

• Clues as to what soil to expect

• Examples:

• Alluvium = particle size based on energy

• Colluvium = mix but expect clay

• Sandstone = sand

• Basalt = weathers to clay

• Terrace gravel = gravel and sand (R-soil but should perc well)

• Glacial till = R-Soil

Case Study: Research and Prospecting = Type 1 vs. Type 4 Soil

Case Study: Fan deposit over alluvium

• Research – what do we expect?

• Look at landforms – check for clues

• Prospect – what do we find?

• Fan deposit = poorly sorted angular particles

• Alluvium = well sorted stratified uniform sand (stuff dreams are made of)

Qal

Qf

Qf

Qal

Case Study: Terrace Gravel over Clay

• Terrace gravel flat lying over older Troublesome

• Forms an armor resistant from erosion

• Higher on the hill = thicker Type 2/R-1 soil vs. Type 4

Qg

NPet

Qg

NPet

Other Clues: Vegitation

• Vegetation• Some plants tolerate clay better (sage, grasses?)

• Pines aspens tend to like sandy soils

• Tqp=quartz monzonite

• Intrusive igneous that can weather to a sandy gravel

• Possible Type R-1 Option 2 scenario

• Qac=colluvium

• likely high clay content here

• Worth prospecting in different vegetation

Other Clues: Rodent Excavations

Other Clues: road cuts, borrow pits, old profile pits

Questions?

Resources:

• Colorado Geological Survey website: http://coloradogeologicalsurvey.org/• Geologic Map of the Keystone Quadrangle, Summit County, Colorado, (Open File Map 02-3) by Beth L.

Widmann, Matthew L. Morgan, Paul J. Bartos, Kenneth C. Shaver, Francisco Gutierrez, and Andrew Lockman of the Colorado Geological Survey, 2003.

• Geologic Map of the Breckenridge Quadrangle, Summit and Park Counties, Colorado (Open Map 02-7) by C.A Wallace, John W. Keller, James P. McCalpin, Paul J. Bartos, Erik E. Route, Natalie N. Jones, Francisco Gutierrez, Cindy L. Williams and Matthew L. Morgan with Colorado Geolgical Survey, 2003.

• Geologic Map of the Fraser 7.5-Minute Quadrangle, Grand County, Colorado (SI Map 3130, Version 1.0), by Ralph R. Shroba, Bruce Bryant, Karl S. Kellogg, Paul K. Theobald, and Theodore R. Brandt with the U.S. Geological Survey, 2010.

• Geologic Map of the Granby Quadrangle, Grand County, Colorado (Map GQ-1763) by Schroeder, D.A. with the U.S. Geological Survey, 1995.

• Messages In Stone – Colorado Colorful Geology – Second Addition; Colorado Geological Survey; Matthews, Vince; 2009.

• Prairie Peak and Plateau – A Guide to the Geology of Colorado – Colorado Geological Survey Bulletin 32; Chronic, John and Halka, 1972.