Skip Links

USGS - science for a changing world

Scientific Investigations Report 2006-5242

Table Mountain Shoshonite Porphyry Lava Flows and Their Vents, Golden, Colorado

By Harald Drewes

Thumbnail of and link to report PDF (18.5 MB)Download Publication
Scientific Investigations Report
2006-5242 PDF (18.5 MB)
Abstract

During early Paleocene time shoshonite porphyry lava was extruded from several plugs about 5 km north of Golden, Colo., to form lava flows intercalated in the upper part of the Denver Formation. These flows now form the caps of North and South Table Mountains. Detailed field and petrographic studies provide insights into magma development, linkage between vents and flows, and the history of the lava flows.

The magma was derived from a deep (mantle) source, was somewhat turbulent on its way up, paused on its way up in a shallow granite-hosted chamber, and near the surface followed the steep Golden fault and the thick, weak, steeply dipping Upper Cretaceous Pierre Shale. At the surface the lava flowed out of several plug and dike vents in a nonexplosive manner, four times during a span of about 1 m.y. Potassium-rich material acquired in the shallow chamber produced distinctive textures and mineral associations in the igneous rocks.

Lava flows 1 (the lowest) and 2 are channel deposits derived from the southeastern group of intrusions, and flow 1 (a composite, multiple-tongued flow) lies about 50 m below the capping flows. Provisionally, the unit termed flow 1 is considered to include older, felty-textured flows that are distinguished from a blocky-textured unit, flow 1a. Flow 2, newly recognized in this study, lies immediately beneath the capping flows. Lava flows 3 and 4, more voluminous than the earlier ones, were derived from a plug vent 1–2 km farther north-northwest and flowed south-southeast across a broad alluvial plain. This plug is a composite body; the rim phase fed flow 3, and the core phase was the source of flow 4. During the time between the effusion of the four flows, the composition of the shoshonite porphyry magma changed subtly; the later flows contain more alkali, as shown by higher proportions of sanidine.

On North Table Mountain, lava flows 3 and 4 form an elongate tumulus above a stream channel that carried water at the time of their eruption. On South Table Mountain, lava flow 3 forms a low, broad dome that forced flow 4 into channels now restricted to the west and northeast flanks of that mesa.

Mesa-capping lava flows 3 and 4 are broken by many small normal faults and are warped into open synclines, probably in response to local stresses associated with the settling of piedmont deposits into the Denver Basin. Mid-Tertiary deposits are inferred to have covered the upper part of the Denver Formation and its lavas; these deposits could thus have been instrumental in changing the stream flow direction to the east before the onset of Neogene uplift and consequent canyon cutting across the flows. Other younger deposits may also have covered the area, to be linked to this consequent canyon cutting.

Version 1.0

Posted April 2008


Suggested citation:

Drewes, Harald, 2008, Table Mountain shoshonite porphyry lava flows and their vents, Golden, Colorado: U.S. Geological Survey Scientific Investigations Report 2006–5242, 28 p.


Contents

Abstract

Introduction

Previous Work and Methods Used in This Study

Overview of the Igneous Rocks

Cretaceous and Tertiary Sedimentary Rocks

Field Observations

Ralston Intrusive Bodies

Ralston Plug

Southeastern Group of Intrusive Rocks

North Table Mountain Lava Flows

Lava Flow 1

Lava Flow 2

Lava Flow 3

Lava Flow 4

Human Use of the Lava Rocks

South Table Mountain Lava Flows

Problems in Interpreting the Lava Flows

Geophysical Data

Petrographic Observations

Textures

Minerals

Mineral Modes

Petrogenesis of the Lava Flows

Summary of Conclusions

Acknowledgments

References Cited

Glossary

 


Document Accessibility: Part or all of this report is presented in Portable Document Format (PDF); the latest version of Adobe Acrobat Reader or similar software is required to view it. Download the latest version of Acrobat Reader, free of charge or go to access.adobe.com for free tools that allow visually impaired users to read PDF files.

Accessibility FOIA Privacy Policies and Notices

Take Pride in America home page. FirstGov button U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubsdata.usgs.gov/pubs/sir/2006/5242/index.html
Questions or Assistance: Contact USGS
Last modified: Thursday, 01-Dec-2016 19:39:01 EST