Laramie Correlation Map of the Denver

Home , Arapahoe Formation

11. Geol. Soc. Am., vol. 67 Reichert

R.5I

Legend

Castle Rock conglomerate

l-\ Castle Rock volcanics ~" •

Oawton arkose (re-defined and extended)

Table Mountain basalt flows

Denver formation (re-defined and extended)

Cretaceous - Poleocene contact—-/ of R.W.Brown(l943,p.67)

Sasal member of Denver formation

Base mop drawn from 1935 geological map of Colorado Arapahoe formation (re-defined and extended) Note: Compare with Plate 2 Correlolion by Stanley 0. Reichert 1955

.oramle formation as originally defined) -LARAMIE CORRELATION MAP OF THE DENVER BASIN, COLORADO POST Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/67/1/107/3441540/i0016-7606-67-1-107.pdf by guest on 02 October 2021 BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 67. PP. 107-112, 2 PLS. JANUARY 1956

POST-LARAMIE STRATIGRAPHIC CORRELATIONS IN THE DENVER BASIN, COLORADO

BY STANLEY O. REICHERT

ABSTRACT In mapping and correlating post-Laramie formations in the Denver Basin, clear-cut mappable formation boundaries have been defined and used. It is proposed that (1) the Laramie-Arapahoe contact remain as originally defined by Emmons, Cross, andEldridge (1896); (2) the Arapahoe-Denver contact be placed at the widespread erosional disconformity at the base of the lowest, thickest, and most prominent basalt-andesite pebble conglomerate bed in the Denver area, instead of at the first appearance of andesitic debris as originally proposed by Emmons, Cross, and Eldridge (1896). The following changes in nomenclature are proposed: (1) The Arapahoe and Denver formations can be mapped throughout the Denver Basin, and these names should replace the term "lower Dawson" of Dane and Pierce (1936). (2) "Dawson formation" (type section at Dawson Butte) should designate only "upper Dawson" of Dane and Pierce. (3) The name "Green Mountain conglomerate" should be abandoned as it is only a partial representative of the proposed Dawson formation as restricted in (2). (4) The Laramie formation should be restricted to its original definition by Emmons, Cross, and Eldridge (1896) rather than extended as proposed by Brown (1943).

CONTENTS

TEXT ILLUSTRATIONS Page Plate Facing page Introduction 107 1. Post-Laramie correlation map of the Denver Basin, Colorado 107 Formations involved 108 2- Post-Laramie correlation of stratigraphic columns in the Denver Basin, Colorado.... 112 Summary of methods and results 108 TABLE Conclusions and proposed changes in nomen- Table Page clature 110 i. Comparison of thicknesses of post-Laramie sediments, Green Mountain area, Jefferson Bibliography Ill County, Colorado 108

INTRODUCTION mittently on detailed mapping of the Golden- The purpose of the present paper is to review Green Mountain area (Reichert, 1954, Pis. 2, the post-Laramie stratigraphy of the Denver 3> and on reconnaissance mappmg of the post- Basin and show that clear-cut mappable bound- Laramie sediments along the perimeter of the aries between formations can be selected over Denver Basin. The small-scale (1 inch = 2 the entire area. Lithologic characteristics are miles) mapping done directly on the U. S. Geo- the basis for the designation of formations. A logical Survey topographic sheets has been revision of the nomenclature is proposed. transcribed onto a base map of the Denver From 1946 to 1955 the writer worked inter- Basin which was traced from the U. S. Geo- 107

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TABLE 1.—COMPARISON or THICKNESSES or POST-LARAMIE SEDIMENTS, GREEN MOUNTAIN AREA, JEFFERSON COUNTY, COLORADO Thickness (in feet) according to: Formation Emmons, Cross, Marr (1929, Colo. School of Mines, Reichert Eldridge Unpub. Rept., p. 11) (1896, PI. 5) Min. | Max. Green Mountain conglomerate 423 525 590 610 Denver 434] 915] 293] 323) [1125 [ms [966 [1067 Arapahoe 691 j 800J 673J 744J

Total post-Laramie thickness 1548 2240 1556 1677

logical Survey 1935 geological map of Colorado. There is disagreement for each individual Plates 1 and 2 show the results of this work and formation thickness wherever the lithologic the correlations proposed. horizon is uncertain. The writer chose prominent lithologic breaks FORMATIONS INVOLVED which could be traced over the entire large- scale aerial-photograph mosaic of the Golden- This paper is concerned only with the post- Green Mountain area (Reichert, 1954, PL 3). Laramie formations in the Denver Basin shown The base of the Arapahoe formation as defined on Plates 1 and 2. In the northern part of the by Emmons, Cross, and Eldridge (1896, p. 154) area the U. S. Geological Survey recognizes the is marked by an arkosic conglomerate 100 feet Arapahoe as the older and the Denver as the thick. It is easily traced on the mosaic and in younger formation. In the Castle Rock area and the field. The base of the Denver formation was southward, these same formations have long placed by Emmons, Cross, and Eldridge (1896, been designated as lower Dawson and upper p. 160) at the first appearance of andesitic Dawson, but exact correlations had not yet debris. This horizon varies and cannot be de- been established when the 1935 geological map lineated on the large-scale mosaic nor on the of Colorado was published, and no attempt was ground. It is unmappable and has led to great made on the Colorado map to differentiate any confusion throughout the Denver Basin. The of the post-Laramie sediments except the writer chose the base of the lowest, thickest, Castle Rock conglomerate. and most prominent basalt-andesite pebble- conglomerate bed as the base of the Denver for- SUMMARY OF METHODS AND RESULTS mation. This bed can be traced on the aerial- photograph mosaic of the Golden-Green In using a semicontrolled aerial-photograph Mountain area. Finlay (1916) mapped this mosaic on the scale of 8 inches to 1 mile for the horizon in the Colorado Springs folio as the field mapping of the Golden-Green Mountain Tda unit of the Dawson arkose, and the writer area in Jefferson County, Colorado, the writer traced it for 10 miles along the strike of the found discrepancies in the total thickness of all beds in the Castle Rock quadrangle (PI. 1). post-Laramie formations as well as in individual The base of the Green Mountain formation formation thicknesses (Table 1). as defined by Hares (1926, p. 175) and Marr Thicknesses in Table 1 were obtained by (1929, Colo. School Mines Unpub. Rept., p. measuring the same section exposed along the 10-13) is marked by an erosional disconformity west side of Green Mountain, SWJ4 Sec. 13, overlain by a basal andesite- and basalt-pebble T. 4 S., R. 70 W. Care was taken to select the conglomerate 50-150 feet thick, which is easily same stratum, as nearly as could be determined traced in the field and on the large-scale mosaic. from Cross' or Marr's description, for the The proportion of basic lava detritus decreases boundary in each case. rapidly up section, being replaced by ever-

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increasing quantities of boulders of Precam- Monument Creek-Denver contact was traced, brian granites, gneisses, schists, quartzites, and intermittently, from 6 miles north of Sedalia to sandstones toward the top of the Green Moun- 8 miles southeast of Falcon. This Denver- tain conglomerate. The lower basalt-andesite Monument Creek or lower Dawson-upper pebble member of the Green Mountain con- Dawson contact is shown on Plate 1 as a broken glomerate thins and disappears southeastward. line so as to distinguish it from its solid-line Southeast of Littleton in T. 6 S., R. 68 W., the continuation, which had been mapped by basal part of the so-called Monument Creek earlier workers. Lastly, R. W. Brown's Creta- formation is an arkosic sandstone and con- ceous-Tertiary time line (Brown, 1943, p. 67) glomerate, 50 feet thick, very similar litho- was transcribed to Plate 1. logically to the basal part of the upper member Plate 2 shows four measured stratigraphic of the Green Mountain conglomerate on Green sections (1, 2, 3, and 4) corresponding to the Mountain, except that the conglomerate be- numbers on Plate 1. The following horizons, comes finer-grained southeastward. Ten miles from older to younger, were used to establish due south, in Sec. 20, T. 7 S., R. 68 W., this correlations: same 50-foot arkosic sandstone and conglom- (1) The erosional disconformity separating erate is overlain by a 50-foot sandy, carbona- the basal Arapahoe arkosic conglomerate from ceous shale containing abundant fossil plants. the underlying Laramie clays These two beds are correlated with beds 7 and 8, (2) The erosional disconformity between the respectively, of R. W. Brown's (1943, p. 73) base of the basal andesite-basalt-pebble con- section at the Douglas (Lehigh) Coal Mine. glomerate and the underlying light-colored Yet here these beds belong to the upper Daw- arkosic, sandy shales of the Arapahoe forma- son formation; south of Littleton they are called tion (This disconformity marks the base of the Monument Creek formation, and on Green redefined Denver formation of this paper and is Mountain they are called Green Mountain correlated with the disconformity at the base conglomerate (Pis. 1, 2). of the Tda unit of the lower Dawson arkose, Before the boundaries were drawn for the shown on the stratigraphic sections at Castle post-Laramie formations (PI. 1), the location, Rock, Colorado Springs, and Calhan.) geologic age, and lithologic characteristics of (3) R. W. Brown's Cretaceous-Tertiary time every place where plant or animal fossils had line been collected were plotted on the map. Next, (4) The prominent erosional disconformity the Denver-Monument Creek boundary, as between the base of the light-colored arkosic given by Emmons et al. (1896, PI. 2), was sandstones and conglomerates of the upper transcribed to Plate 1 as a solid line from Scran- Dawson and the underlying darker-colored ton to 6 miles north of Sedalia. The lower graywacke, andesitic, micaceous, carbonaceous Dawson-upper Dawson contact, as mapped by sandstones and shales of the lower Dawson Dane and Pierce (1936, p. 1314-1315), was (In the northwestern part of the Denver Basin drawn as a solid line from 8 miles southeast of this horizon is the Green Mountain-Denver or Falcon to 13 miles west of Bland. The basal the Monument Creek-Denver boundary.) Dawson Tda unit, as mapped by Finlay (1916, (5) R. W. Brown's Paleocene plant horizon geologic map), was next drawn as a solid line Using these five horizons for correlation the from 3 miles west of Drennan to 3 miles north- following facts are noted and mapped: west of Pikeview. The continuation of this Tda (1) The Arapahoe formation is thickest in the unit, as mapped by the writer in 1954 along the Golden-Green Mountain area. The source of unfaulted Laramie-lower Dawson contact from these elastics is thought to have been the rising 5 miles west of Larkspur Butte to 5 miles north- Front Range in late Cretaceous time. The west of Dawson Butte, was plotted next on Arapahoe thins and becomes finer-grained to Plate 1. The Arapahoe-Denver contact, as re- the east, south, and southeast. Only the basal defined in this paper, was traced from 4 miles conglomerate or coarse arkosic sandstone is northeast of Morrison to 4 miles southwest of found east and southeast of the Castle Rock Sedalia, where it is faulted out. Similarly, the quadrangle, where it ranges from 0 to 35 feet

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thick in the Castle Rock and Colorado Springs west of Sedalia, near Brown's (1943, p. 73) areas, and from 50 to 100 feet thick in the measured section. A complete section of the Matheson-Bland-Byers areas. This basal sand- Dawson arkose from the Laramie-Arapahoe stone and conglomerate characteristically con- contact up to the rhyolite lava flow capping tains small pebbles of black, red, and yellow Dawson Butte was measured in Sections 1 and chert, white limestone, and granite in a matrix 12, T. 9 S., R. 68 W. (Photographs of these of coarse arkosic yellow sandstone. The out- rocks in the Dawson Butte area are shown in crop width of this bed (Ka) is too small to map Reichert, 1954, p. 18, 20, 48.) The type section to scale along the southwest and east edges of of the upper Dawson of Dane and Pierce (1936, the Denver Basin, so is exaggerated in Plate 1. p. 1320-1325) is shown as column 2 of Plate 2, (2) The Denver formation is also most char- and the type locality of the upper Dawson is on acteristically developed in the Denver-Golden- the west flank of Dawson Butte in the Castle Green Mountain area, where it is typically Rock quadrangle. composed of andesitic and basaltic sandstones, shales, and conglomerates. Like the Arapahoe, CONCLUSIONS AND PROPOSED CHANGES it changes composition and becomes finer- IN NOMENCLATURE grained eastward and southeastward away (1) The Arapahoe formation is the lithologic from the Front Range volcanic area. The basal and time equivalent of the basal part of the conglomerate (Tda) member in the Golden- lower Dawson of Dane and Pierce in the south- Green Mountain area can be traced as an an- east part of the Denver Basin. Only the basal desitic sandstone bed overlying the light-colored conglomerate or coarse arkosic sandstone is Arapahoe in the Castle Rock and Colorado found throughout the Denver Basin. Since an Springs areas. This same bed is believed to erosional disconformity everywhere separates occur as a dark-green and brown graywacke the Arapahoe from the overlying (Tda) basal sandstone and shale (Tda) in the Matheson- member of the Denver formation, variations in Bland-Byers areas. The dark color and abun- the original thickness of the Arapahoe and pre- dant detrital hornblende, biotite, augite, and Denver erosion account for the great variation plagioclase established its correlation with the in the thickness of the Arapahoe. The Arapahoe Tda member of the Denver formation in the formation throughout the Denver Basin should Denver-Castle Rock-Colorado Springs areas. include all Cretaceous light-colored arkosic Lithologically it is quite distinct from the conglomerates, sandstones, and shales from the light-colored arkosic sandstone and shale of the unconformity at the top of the Laramie up to Arapahoe. The Tda member of the Denver for- the dark andesitic or graywacke conglomerates, mation varies from a 10-foot andesite-basalt sandstones, and shales of the Tda member of pebble conglomerate in the Golden area, to a the Denver formation as defined and mapped 30-foot andesitic sandstone in the Castle Rock in this paper. Therefore, it is proposed that the area, to a 90-foot andesitic sandstone in the Colorado Springs area, to a 10-50-foot dark term Arapahoe be extended to cover the basal conglomerate (or remnants of the basal graywacke sandstone and shale in the south- Arapahoe conglomerate) throughout the Den- eastern and eastern parts of the Denver Basin. ver Basin as shown in Plates 1 and 2. Its outcrop width on Plate 1 is, therefore, (2) The Denver formation is the lithologic exaggerated. and time equivalent of that portion of the lower (3) The Dawson arkose as originally de- Dawson of Dane and Pierce in the southeast scribed and defined by Richardson (1915, p. part of the Denver Basin extending from the 7-14) attains a maximum thickness of about base of the Tda unit up to the widespread ero- 2000 feet in the Castle Rock quadrangle. This sional disconformity, which is marked by the section was remeasured by the writer and is abrupt change from dark graywacke, carbo- shown as column 2 on Plate 2. The lower 1200 naceous sandstones, and shales to light-gray, feet from the basal Arapahoe conglomerate to coarse-arkosic sandstones and conglomerates. above the Paleocene plant horizon was meas- As seen in Plates 1 and 2, Brown's Cretaceous- ured in Section 30, T. 7 S., R. 68 W., 4 miles Paleocene paleontologic time line varies in

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stratigraphic position with respect to the base was deposited during a short period of time, and and top of the Denver formation. It is proposed tectonic conditions fluctuated rapidly causing that the Denver formation be extended large differences in lithology and widespread throughout the Denver Basin to include all erosional disconformities. dark clastic sediments derived from the erosion (7) Paleontologic time lines diverge from of basic lavas as indicated by their mineral formation boundaries because of the complex composition, which is in strong contrast to the tectonic control of sedimentation in the Denver light-colored arkosic sediments in the under- Basin in post-Laramie time. lying Arapahoe and the overlying upper Daw- son of Dane and Pierce. Plates 1 and 2 show the BIBLIOGRAPHY Arapahoe and Denver formations as extended, Brown, R. W., 1943, Cretaceous-Tertiary boundary with the elimination of the term "lower Daw- in the Denver Basin, Colorado: Geol. Soc. son" of Dane and Pierce. America Bull., v. 54, p. 65-86. (3) Since the Green Mountain conglomerate Dane, C. H., and Pierce, W. G., 1936, Dawson and Laramie formations in southeastern part of is only a partial representative of the type sec- Denver Basin, Colorado: Am. Assoc. Petroleum tion of the upper Dawson, as are also the upper Geologists Bull., v. 20, p. 1308-1328. Dawson sediments in the Colorado Springs and Emmons, S. F., Cross, C. W., and Eldridge, G. H., 1896, Geology of the Denver Basin: U. S. Geol. Calhan areas, it is proposed that the terms Survey Mon. 27, p. 1-556. "lower Dawson" and "upper Dawson" of Dane Finlay, G. I., 1916, Description of the Colorado Springs quadrangle, Colorado: U. S. Geol. and Pierce and "Green Mountain conglomer- Survey, folio 203. ate" of Hares (1926, p. 175) be eliminated and Hares, C. J., 1926, What is the Denver formation? be replaced throughout the Denver Basin by (Abstract): Geol. Soc. America Bull, v. 37, p. 175. Arapahoe, Denver, and Dawson formations as Lavington, C. S., 1941, Is there an unconformity shown in Plates 1 and 2. between the Denver formation and the Dawson (4) The Tda basal member of the Denver arkose? Paper presented before Rocky Moun- tain ASEOC. of Geologists. formation is shown as a separate unit on Plates LeRoy, L. W., 1946, Stratigraphy of the Golden- 1 and 2, mainly to establish the stratigraphic Morrison area, Jefferson County, Colorado: Colo. School Mines Quart., v. 41, no. 2, p. 1-115. correlations, but a revised geological map of Reichert, S. p., 1954, Geology of the Golden-Green Colorado need not make this distinction. Mountain area, Jefferson County, Colorado: (5) In view of the limits set for the Arapahoe, Colo. School Mines Quart., v. 49, no. 1, p. 1-96. Richardson, G. B., 1915, Description of the Castle Denver, and Dawson formations, the Laramie Rock quadrangle, Colorado: U. S. Geol. Survey, formation should be restricted to its original folio 198. definition and usage by Emmons, Cross, and LOUISIANA STATE UNIVERSITY, BATON ROUGE, LA. Eldridge (1896, p. 72-77). MANUSCRIPT RECEIVED BY THE SECRETARY OF THE (6) A thick section of post-Laramie elastics SOCIETY MARCH 10,1955

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,_ 0 0 o. 55 o" roposed Note; See Plate I for map locations. Q. 2> 0 JO* s Castle Rock g 3 — u Quadrangle ^^ 00 h- •s • — O O 8 200' Ul 2 rr light-colored arkosic i at ^1 co tn sandstone and conglomerate 100' § 8" 1 a '•§ o c O rhyolite conglomerate E ilMuiul rhyolite lava flow 0 + -' rhyolite tuff Vertical Scale JaC 1£ 6 rr 1 light- colored arkosic _?- Green Mountain, conglomerate, sandstone, | TJ near Golden 12 ^^ * and sandy shale

u « 1 c | § s o Q c Southeast part •s oJ Q £ of Denver Basin, o '^^.—^.'^f Paleocene plant horizon (R.W. Brown, 1948) c near Calhan c i ~- —-~ (eee Reichert,I954,p.27) !i 1 a> 0 u Colorado Springs e 0 ••.• »••.».•..«. .j/ f plant horizon 2 miles ^~^>^ j^( Paleocene plant horizon (bed 8,Brown, 1943,P.73) Quadrangle Q | £ /•TYaTkose (bed_7,Brown,l943,p.73) S.E. of Calhan(D.ond P.,I936,P.I3I5) y Table Mountain basalt light-colored arkoeic sandstone S a> C~ondesitic sondsfoTir _and_ cojiglomerate. _ S •o lava flows —{ light "colored" sandy'shale" •5 £ H Paleocene plant horizon (Brown, 1943, p. 77) ^t^ff-l andesite boulders unexposed thin lignitic coal o —Tl graywacke shale — near Falcon. —-" c I W micaceous sandstone and shale c : light-colored arkosic sandstone and conglomerate, containing basal Fbleocene plants °j > 'AVJ.^KI.V.^Xondesitic sandstone Tertiary .•_•'.•.•.*. ••/.•) light- colored orkosic conglomerate (Dane and Pierce,I936,pp.l3l5,l324) •; a caprock at Corral Bluffs 1 • " ' ' Cretaceous Teddish and dark shale (bed 5,Brown, 1943,p.73 ) '\J8row_n ,^943,^67,76) 3 0 a — i and basalt conglomerate (Tda) member dark"colored sandy shale O» 1 containing dinosaur bones c jy i unexposed light-colored sandstone and shale (Dane and Pierce,I936,p.l328) t < '•a containing Poleocene mammals (Brown, 1943, p. 76) dark-colored carbonaceous and c andesitic sandstone and shale containing lignitic shales and sandstones c Hornblende - Aug/ite Phase groywocke shale dinosaur bones (Brown, 1943, p. 76 ) < QJ Q -3^t gray,tan,buff sandstone, "andesltlc sandstone (Tda) member i C dark-colored sandy graywacke 'i 0) shale, thin conglomerate O light- colored sandy shale shales 5 I unexposed Larami e ia o o a. light-colored sandstone and shale o Cretaceou s o dark-colored orkotic sandstone i < < Flint-Chert Phase 5_20j;ojiglojnecfl)e_composed_of -granite, schist • ke sandstone and shale (Tda) i a a k 90' basal Denver andesitic sandstone ( Tda) member i an c 1 member — — Ol 100' basal c 1100' basal Arapahoe sandstone D and conglomerate. (Ko) < 58 iSiS/tiiSiJ _3Q_]_basfll_AraQflhflfi '•^i^^^^ 0-35' basal Anjpahoe conglomerate (Finloy,|9|6,p.9) (Don* and Pierce, 1936,p. 1321) Laramie Laramie Laramie Loramie

POST-LARAMIE CORRELATION OF STRATIGRAPHIC COLUMNS IN THE DENVER BASIN. COLORADO

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