T-1947
A SEISMIC INVESTIGATION OF NORTH AND SOUTH
TABLE MOUNTAINS NEAR GOLDEN, JEFFERSON COUNTY, COLORADO
b y
Terence K. Young
1977 ARTHUR CAKES LIBRARY COLORADO SCHOOL of MINES DOLDEN, COLORADO 8Q4Qi ProQuest Number: 10782113
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A Thesis submitted to the Faculty and the Board of
Trustees of the Colorado School of Mines in partial ful fillment of the requirements for the degree of Master of
Science in Geophysical Engineering.
Signed: Student
Golden, Colorado
Date: /? , 19 7 7
Approved: Thesis Advisor
ead of^Department
Golden, Colorado
Date: j % 19 ”7 7
o d e g ;:i^ ? o rrnrorv •_ v in e s •inonovv, T-1947
ABSTRACT
A multifold seismic reflection survey was conducted on
North and South Table Mountains near Golden, Colorado. Using a vibratory surface source, sufficient energy was transmitted through the igneous caprock to produce good quality reflec tion sections. Generally, these seismic sections exhibit a higher frequency content but contain the same prominent geologic reflectors as those sections obtained locally by others working on sedimentary terrain. The high frequency character is presumably the result of three factors: vibra ting on a high velocity (rigid) material, the enhancing of approximately 34 Hz energy by constructive interference in the lava, and low-cut filtering during processing. The data reveal the low-dipping attitu6de of Denver basin sedi mentary layers beneath the lava caprock. There is no evidence in these data to support the geologic hypothesis that an igneous source vent exists under North Table Mountain. Two significant vertical faults, one beneath each mountain, are identified. Both faults extend from the basement up through the Pierre Formation, but they do not penetrate the surface.
Although basement tectonics play a dominant role in the
North Table Mountain faulting, variations in the displacement of key horizons across the fault suggest that recurrent movement or growth has occurred. Uniform displacement across the South Table Mountain fault at all reflectors above the
iii Niobrara implies a single-movement episode in Late Cretaceous resulting from basement uplift, perhaps along a plane of previous faulting. The North Table Mountain fault is the apparent southward continuation of a basin-margin fault previously mapped in the Leyden and Applemeadows areas.
The South Table Mountain fault, which is looated near a high- angle reverse fault previously identified by surface geologic study, is interpreted as projecting northeastward into the
Denver basin.
iv
~ “■ TjAKHS library. C O LC lL'i o SCHOOLol M IN K T-1947
CONTENTS
Page
ABSTRACT iii
LIST OF FIGURES vi
ACKNOWLEDGMENTS viii
INTRODUCTION 1
METHOD OF INVESTIGATION 9 Field Procedures 9 Computer Processing 13
GEOLOGY OF THE STUDY AREA 22 Stratigraphy 22 Structure 25
SEISMIC INTERPRETATION 31 Correlation 31 Discussion 4 2 Alternate Interpretation 47
CONCLUSIONS 51
REFERENCES 52
v T-1947
LIST OF FIGURES
Page
1. Location map of study area 2
2. Dynamite seismic record from No. Table Mtn. 6
3. Theoretical problems with seismic survey over 7 igneous surface material
4. Seismic line location map 10
5. Stacking diagram for So. Table Mtn. 11
6. Crosscorrelation record from So. Table Mtn. 14
7. Brute stack section of So. Table Mtn. 16
8. Brute stack section of No. Table Mtn., Line 1 17
9. Brute stack section of No. Table Mtn., Line 2 18
10. Final section of So. Table Mtn. 19
11. Final section of No. Table Mtn., Line 1 20
12. Final section of No. Table Mtn., Line 2 21
13. Generalized stratigraphic column 23
14. Upper Cretaceous seismic marker horizons column 24
15. Regional fault location map (south) 27
16. Structure cross section of regional faulting 28
17. Regional fault location map (north) 29
18. Geologic section of the Table Mountains 30
19. Interpreted final section of So. Table Mtn. 32
20. Interpreted final section of No. Table Mtn., 33 Line 1
ARTHUR- LAKES LIBRARY T-1947
LIST OF FIGURES (cont.)
Page
21. Interpreted final section of No. Table Mtn., 34 Line 2
22. Geologic section of So. Table Mtn. 36
23. Geologic section of No. Table Mtn., Line 1 37
24. Geologic section of No. Table Mtn., Line 2 38
25. Fault location map 3 9
26. Displacement profile for No. Table Mtn. fault 46
27. Composite structure cross section of regional 48 faulting T-194 7
Acknowledgments
I am grateful to my thesis advisor, Dr. Thomas L.
Davis, for his guidance throughout all phases of this research. His energetic assistance in the field and interpretative insights were particularly helpful.
George Beggs was my tutor and troubleshooter during the data processing portion of this study; his patient assistance is greatly appreciated.
Dr. Phillip R. Romig was instrumental in my decision to study geophysics and has continued to provide much appreciated encouragement along the way.
Permission to conduct this survey on the Table Moun tains was granted by the Adolph Coors Company (Mr. Bill
Moses), Western Paving Construction Company (Mr. Bob Wolf and Mr. "Mac" Graham), and Mr. Leo Bradley. I am grateful to each for making this study possible.
Most importantly, my wife, Nadine, and two sons,
Gabriel and Luke, have given constant behind-the-scenes support and encouragement; and (to steal a line from Robert
Frost) "that has made all the difference". The Continental Oil Company,Seismograph Service Company, and Texas Instruments provided the equipment used in this study to the Colorado School of Mines.
viii
/ 1
T-1947
INTRODUCTION
North and South Table Mountains are a pair of mesas, separated by Clear Creek, which are located on the outskirts of Golden, Colorado (Fig. 1). These large, flat-topped geologic features, on the westernmost margin of the Denver basin, stand in marked contrast to the more rugged and irregular Front Range mountains to the west and the flat- lands to the east. Structurally, the Table Mountains consist of layered sediments of Upper Cretaceous and
Tertiary age, capped by an erosionally resistant series of Paleocene lava flows. North and South Table Mountains have long been the focus of local geologic attention from the standpoint of their genetic association with other igneous features in the region. Recent interest in the
Table Mountains centers around the nature and extent of underlying faulting and possible relationships with other known faults in the area.
Early geologic investigations of North and South Table
Mountains are described in Monograph 27 of the United States
Geologic Survey (Emmons, Cross, and Eldridge, 1896). Numer ous authors such as Johnson (1925, 1930, 1934), Lovering and Goddard (1938), Reichert (1954), and Van Horn (1957,
1972) have included descriptions of the Table Mountains in their regional investigations. Waldschmidt (1939) and
Ahmad (1971) specifically focused on these structures in
ARTHUR LAKES LIBRARY COLORADO SCHOOL of MINES PDLDEMb COLORADO BQ4Q2 T-1947 N /K
LONGMONT
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BRIGHTON \ X DENVER FRONT
RANGE
TABLE GOLDEN MOUNTAINS DENVER IDAHO / SPRINGS „
0 10 i H H Hi. HfT=3 miles
Figure 1• iy&p showing location of Table Mountains 3
T-1947 their respective works. Ahmad's work represents a synthesis of previous goelogic study of the Table Mountains, with additional detailing of the petrology. Thus the literature contains numerous detailed descriptions, based on surface observations, of the stratigraphy and structure of these mountains.
The study described here had a threefold purpose:
(1) to demonstrate the feasibility of acquiring seismic reflection data through a surface layer of igneous rock;
(2) to extend the subsurface seismic mapping recently done by others in the area; and (3) to acquire subsurface data that might answer the question of an igneous source vent under North Table Mountain.
Geologists studying the Table Mountains have formulated various hypotheses regarding the origin of the lava caprock and its genetic relationship to other igneous structures.
Eldridge and Cross each note that a basaltic extrusion onto the shallow Denver sea floor was originally a continuous sheet, which later became divided by the erosive channeling of Clear Creek (Emmons and others, p. 83 and 285).- Marvine correlated igneous material on Green Mountain with the
South Table Mountain caprock (Hayden, 187 3 i n Cross, 18 96, p. 158). LeConte (1868, in Cross, 1896, p. 156) is reported to have believed that the source channel of the extrusion is indicated by "elliptical ponds" on North Table Mountain
ARTHUR LAKES LIBRARY COLORADO SCHOOL of MINES GOLDENb COLORADO 8Q4QI 4
T-1947 which were once craters. However Cross and Eldridge (Emmons and others, 1896, p. 102, 284) theorize that the Table Moun tain lava is genetically associated with the Ralston "dike", which is what remains of the original igneous vent.
Waldschmidt (1939, p. 44) suggests, too, that the lava was extruded from a source associated with the Ralston intrusive.
Johnson (1925; 1930; 1934, p. 21-2) and Ahmad (1971, p. 88) both postulate the presence of an igneous source vent under
North Table Mountain, from which the four flows of mafic latite were extruded.
Prior to this study, known faulting in the Table
Mountains has been limited to a small, high-angle reverse fault on the northern rim of South Table Mountain. This fault is described by Waldschmidt (1939, p. 41-2), Van
Horn (1957, 1972) and Ahmad (1971, p. 33) among others.
Geologic study by Weimer (1973) of faulting in the region surrounding the Table Mountains has stimulated numerous seismic reflection surveys, including Davis (1974), Shuck
(1976), and Money (1977). From these studies, Davis and
Weimer (1976) conclude the existence of two major types of Late Cretaceous faulting along the west flank of the
Denver basin: (1) basement-controlled vertical faults of early Laramide origin, and (2) shallow listric normal growth faults.
ARTHUR CAKES LIBRARY COLORADO SCHOOL of MINES. GOLDEN, COLORADO 8D4Q1 5
T-1947
In 1972 a previous seismic survey on North Table Mountain, using a small explosive-type source, was attempted by Colorado
School of Mines investigators. The fact that this survey did not yield high quality seismic records (Fig. 2) was not surprising because of several known theoretical obstacles.
Figure 3 is a simplified geologic cross-section of North
Table Mountain, depicting these theoretical problems. The reflection coefficient of -0.48 at the first igneous/sedi mentary interface is representative of a very strong reflec tor (Sheriff, 197 5, p. 133). Therefore it is expected that a relatively large percentage of seismic energy will be reflected back to the surface from the first boundary. Of the energy that is transmitted beyond this first reflector, another large fraction will be reflected at the next inter face (Denver-lava), and a great portion of this energy will very likely reverberate in the Denver Formation stratum between the two boundaries of large reflection coefficient.
Furthermore, the irregular surface conditions, which in places consist of bare, hard rock, make it difficult to obtain good coupling of the source with the ground and the geophones with the ground. Sheriff (197 5, p. 126-7) addresses these problems in his paper concerning seismic amplitudes. T-1947 6
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Figure 2. Dynamite record from No. Table Mtn* T-1947 iue hoeia polm prann oa seismic a to pertaining problems Theoretical Figure Source uvy vra inos ufc layer. surface igneous an over survey k In o a «*r‘ CJ uv o *fsj CO a £ bf* >£• F . tn ««. - 'H C-i $ 3 S ) V § \[ K « o W o Q Vl
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T-1947
Notwithstanding these theoretical difficulties, remark ably good quality multifold reflection data were obtained in this survey. The resulting seismic sections show the prominent geologic reflecting horizons from the top of the
Pierre Formation down to the Precambrian basement. The contrasting character of the raw seismic sections obtained in this research (Fig. 6) versus those acquired in the 1972 survey (Fig. 2) is a result of differences in the field parameters utilized. Low-energy, high-frequency explosives — blasting caps and boosters buried two to six feet were utilized in 1972, as opposed to the large vibrating surface source, sweeping 4 8 to 8 Hertz, used here. Geophone station spacing in the earlier survey was about 2 0 feet, in contrast to the 2 00-300 foot spacing employed in this work. Both surveys employed progressive shot-point locations to yield
12-fold data, and both used recording sample intervals of two milliseconds. Generally, the resulting 1972 data is characterized by higher average frequency content (greater than 40 Hz versus about 3 0 Hz) and shallower penetration than the data acquired in this investigation.
ARTHUR LAKES LIBRARY COLORADO SCHOOL of MINES o o ld e M b camb a e k i li i i i r 9
T-1947
METHOD OF INVESTIGATION
Field Procedures
Three lines of seismic reflection data were obtained on the Table Mountains, using equipment belonging to the
Colorado School of Mines. The Vibroseis (trademark of the
Continental Oil Company) source, Geospace geophones, and
Texas Instruments DFS 10,000 digital recording system utilized are the same as those described by Shuck (197 6, p. 14-17).
Figure 4 is a map of North and South Table Mountains, showing the locations of the seismic lines: two on North
Table Mountain and one on South. The distance between geo phone groups on South Table Mountain was 20 0 feet, whereas the geophone group interval on North Table Mountain was 3 00 feet. It was hoped that the strong surface wave recorded on South Table Mountain might be at least partially attenu ated by the longer interval used on North Table Mountain
(i.e., spatial filtering: Born, 1935 in Dobrin, 1960, p.
113). Alternate geophone stations were utilized as vibrator points. Due to limiting factors such as surface obstacles and vibrator malfunctions, up to elevenfold data were actually acquired. Figure 5 is the stacking diagram showing subsurface coverage obtained for South Table Mountain. T-1947 R . 7 0 W . 10 U) ^ U)
Miles Figure 4. Seismic line location map
ARTHUR HAKES LIBRARY COLORADO SCHOOL of MINES POLDEN, COLORADO flftSflfi T-1947 11
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Between 24 and 2 8 sweeps were generated at each vibrator point. There was a vibrator move-up distance of five to ten
feet between each sweep. Hence the source point was actually a linear pattern whose length was approximately the same as the geophone group interval spacing (i.e., 200 or 300 feet).
The 24 to 28 records from each vibrator point were later summed in the computer into one record per vibrator point.
A 48 to 8 Hertz downsweep of eight seconds duration was the input signal. Listening time following the sweep was three seconds, for a total record length of 11 seconds.
The sample rate was two milliseconds in the field, resampled to four milliseconds in processing. Filters used in record ing included a high-cut set at 4 0 Hz (slope 2 4 dB/octave), and a low-cut set at 12 Hz (slope 36 dB/octave). Fixed gain was set at 104 dB.
Some experimentation was done with signal and recording parameters. For example, one vibrator point on South Table
Mountain was recorded with a 75 to 15 Hz downsweep, using high and low-cut filters set at 7 0 and 17 Hz. Filtering parameters of 52 Hz (high-cut) and 10 Hz (low-cut) were utilized for part of the North Table Mountain recording, with final gain adjusted between 100 and 106 dB, depending upon ambient noise. None of these variations in parameters enhanced record quality significantly. 13
T-1947
Computer Processing-
Field data were recorded on 21-track magnetic tapes, which were subsequently processed on a Raytheon 7 03 Phoenix
(trademark of the Seismograph Service Corporation) mini computer. The first processing step was that of demulti plexing: reformatting field tapes from time-sequential to
Phoenix trace-sequential format. Secondly, the summing of multiple sweeps per vibrator point was done as mentioned earlier. Next the records were crosscorrelated with the input sweep signal to yield the reflection data. Static corrections were applied to the data and references to a sloping near—surface datum, generally paralleling the topography. Vibrator-point traces were then regrouped into common-depth-point gathers, as demonstrated in the stacking diagram of Figure 5. At this point in the processing, several analysis pro grams were run on the data to determine the best parameters to apply in subsequent steps. The programs included velocity, filter, and frequency analyses, power spectra, and auto- correlograms. The most challenging aspect of the data processing phase of this research concerned the elimination of the very high— amplitude surface wave evident on the crosscorrelation record of South Table Mountain (Fig. 6). This was ultimately Figure 6. Gr©ssc©rrelati@n record from South Table Mtn. showing high-amplitude sur face wave recorded. 15
T-1947 accomplished by a combined program of frequency filtering and muting. The relatively high-amplitude, low-frequency, low-velocity (steep slope) appearance of the ground roll stands out in Figure 6. One can see that the muting pattern had to be extended in time to almost one second on the far traces when vibrating off the end of the spread. This, of course, resulted in stacked records with the first 20 0-30 0 milliseconds of data removed. One might also note (Fig. 6) the first arrival wave of about 10,000 feet-per-second velocity, which corresponds to shear wave velocities in basalt obtained by others (Press, 1966, p. 198).
The records were corrected for normal moveout. Auto matic statics were applied with subsequent stacking of the data. Trace-equalization trim was applied several times to enhance portions of the seismograms which had lower ampli tude data due to lower-fold recording (i.e., the ends of the records). Finally, the stacked records were referenced to a flat datum of 5800 feet elevation for interpretation.
"Brute stack" sections (normal moveout corrected, muted, and stacked) for the three lines are given in Figures
7, 8, and 9. The final stacked sections (automatic statics applied and referenced to the flat 5800 foot datum) are presented in Figures 10, 11,and 12.
ARTHUR CAKES LIBRARY COLORADO SCHOOL of MINES GOLDEN, COLORADO 8Q4QI T-1947
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Figure 7» Brute a "tack section of* South Teble Mtn* Figure 8» Brute stack section of North Table Mtn. Line T-1947 18
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Figure 9- Brute stack of North Table Mtn. Line 2 -1947 1,3 11
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Figure 10. Final sectien ©f S©uth Table Mtn. T-1947 20 3 5 7 9 11 13
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Figure 11* Final section ©f North Table Mtn. Line 1
ARTHUR CAKES LIBRARY COLORADO SCHOOL of MINES GOLDEN, COLORADO BQ4Qj T-1947 21
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Figure 12* Final section of North Table Mtn. Line 2 22
T-1947
GEOLOGY OF THE STUDY AREA
S tratigraphy
The stratigraphy of the Table Mountains is detailed by many investigators, including Cross (Emmons and others, 1896, p. 161-170) and Ahmad (1971, p. 16-19). The Table Mountains are a particularly good location for studying the non-resistant
Denver Formation, which has been protected in this locality by the competent caprock. The individual lava flows are also described by Cross (Emmons and others, 1896, p. 285-298),
Ahmad (1971, p. 41-57), and by Waldschmidt (1939, p. 16-20).
The general stratigraphy of the Golden-Morrison area, after
LeRoy and Weimer, is given as Figure 13 (LeRoy & Weimer,
1971). Based on the work of Griffits (1949), Davis (1974, p. 6) presents an Upper Cretaceous stratigraphic column which includes intraformational horizons that are key seismic markers in the study area. Davis' column is given as Figure
14. A specific paper on the Hygiene sand member of the
Pierre Formation is given by Porter (197 6). Moredock and
Williams (1976) discuss both the Terry and Hygiene sand stones as petroleum producers. Shuck (1976, p. 9-11) and
Money (1977, p. 10-16) discuss the local stratigraphy in terms of its ability to be differentiated seismically. The key lithologies identified seismically in this study are 1947 23
Age Formations Graphic section Symbol
Green Mountain und if f e rent ia t ed Conglom ero te D e n v e r - seismically Arapahoe Fm.
La ram ie Fm.
P ie rre Fm, Kp
N io b r a r a Fm. Kxi Figure 1^. Generalized stratigraphic sectien, Golden-Morrison area Kb (after LeRoy and Weimer, 1971)
South Platte
M o r r is o n Fm.
Ralston C reek Fm.
L ykins Fm. undifferentiated 7 seismically Lyons Fm.
Fountain Fm.
J PRf CAMBRIAN PC DENVER-ARAPAHOE ( KTda) LARAMIE (Kl) FOX HILLS(Kf)
TRANSITION MKR a O ( T Z 1) £ P uj to £ L Z TRANSITION MKR' to -M a: < ( T Z2) 1 L l I q : — j— oO o_ o LARIMER& ROCKY RIDGE
to ZD OL
NIOBRARA (Kn)
BENTON
Figure 14. Upper Cretaceous seismic marker horizons (from Davis, 1974) 25
T-19 4 7
the Pierre Formation (including the top boundary, transition markers one and two, the Terry sand, the Hygiene sand and
base of the Hygiene zone), the Niobrara Formation and the
Dakota Group. Additionally, a prominent reflector - pre
sumably a sand layer - stands out between the top boundary
and first transition marker of the Pierre. It is denoted
here by the symbol " KpS]_" •
Structure
Weimer (1973) has integrated key stratigraphic and
structural observations of his own with those of other
investigators to develop a comprehensive model of the upper most Cretaceous paleoenvironments. The result is a systematic,
definitive framework within which much of the heretofore
uncorrelated geological data of the region clearly fits.
Weimer's observations of syndepositional slump and growth
fault features and his determination of the existence of a
basement-controlled basin-margin fault system along the east
flank of the Central Front Range have been subsequently
documented by the seismic reflection studies of Davis (1974),
Shuck (1976), and Money (1977). A compilation of the results
of these investigations is given in a paper coauthored by
Davis and Weimer (1976). Faulting in the vicinity of the
Table Mountains, then, has been shown to include: (1) the 26
T-1947
high-angle reverse-type Golden Fault complex; (2) a zone of basement-controlled flank deformation consisting of sever air*-**-”* vertical faults along which some recurrent movement or growth occurred; (3) shallow, listric normal growth faults. Figure
15 is a map and Figure 16 a vertical section, both after
Weimer (1976, p. 189, 201), showing fault systems west and south of the Table Mountains. Figure 17 is a map modified
from Money (1977, p. 51), depicting the faults mapped by
Money and by Shuck (1976) just north of the Table Mountains.
It has been previously noted that both Waldschmidt
(1939, p. 41-2) and Ahmad (1971, p. 33) describe a high- angle reverse fault on the Clear Creek side of South Table
Mountain (SW 1/4, Sec. 26, T3S, R70W). Waldschmidt suggests projecting the fault in a northwesterly direction to tie with possible faulting indicated in lava flows of North Table
Mountain. Ahmad maps the same fault for only a short dis tance, in a trend just east of north. Van Horn (1957, 1972) shows this fault in an even more northeasterly trend, missing
North Table Mountain altogether.
One further structural feature postulated by Johnson (1934, p. 21-2) and Ahmad (1971, p. 88) is the possible existence of an igneous vent under North Table Mountain. Figure 18 is a cross-section through North and South Table Mountains, after
Ahmad, showing this feature.
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Figure 15* Fault location map (after Weimer* 1976 p.169)
I \ T-1947 28
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JM I u m 4he Table Mountains (after 1 Abma CO 3: K O <0 ml Figure Figure 18. Geologic Section 31 T-1947 SEISMIC INTERPRETATION Correlation Two methods were used in this study to correlate seismic horizons with stratigraphic units. First, the character (seismic signature) of each reflector was compared with other previously correlated seismic data from the Golden area, en abling a tentative identification. Secondly, this preliminary identification was checked against the stratigraphic depth information obtained from electrical well log data: Johnson #1, Farmers Highline and Canal Reservoir Co., SE SE NW Sec. 7, T.3S., R.69W. The resulting identification of stratigraphic horizons is shown on interpreted seismic sections in Figures 19, 20, and 21. Using interval thickness information derived from well data, computer derived velocity analyses, and the seismic time section intervals, seismic interval velocities were cal culated. These velocities are the key to conversion of the seismic time sections to geologic depth sections. Because these calculations were done in collaboration with Money, the jointly derived results tabulated by Money (1977, p.32) are reproduced here (Table 1). Figures 22, 23, and 24 are geolo gic sections constructed along the three seismic lines (whose locations were given in Figure 4). A map view of interpreted faults is given in Figure 25. ARTHUR CAKES LIBRARY* .COLORADO SCHOOL of MINES GOLDEN, COLORADO 8Q4QB T-1947W 32 DEPTH 2-WAY TIME (FT) (SEC) DATUM 580 o' 1Optfp llr C'-vby.'Z V C ;»A *-vI* ,»rw, C I - 3 OOOH 0 - 4 5 0 0 - . lf* !**■»*»; |«|. » - 7 5 0 0 - U > V i* ■•> <-*,: M* f£ . V .t«r* frx f. ‘*\ ^, .*• *«■ - 4 o o o - c . ^V.ik-“> v <1 > !l» !* / V > > • . 1 0 500 - Figure 19* Interpreted final section ©f South Table Mtn. N T-1947 33 DEPTH 2-WAX TIME (FT) (SEC) d a t u m !s=^i|K >'r vf' > > : ini a f t *|op ICp ■fc ^v ' KfsA ><.>>• |^a ’ a^'ft > ?*-». > $ &*£ I*. T Z 1 3 6 o O - \ /,■•*,!*:«:>jfVh CBr^ V n r ^ a » fcV i M > I. *>>>.'* TZZ • ( W & m kn K*> - - * 1 0 5 0 0 - - n a o o o - p * L.5.._ Figure 20. Interpreted final section of Nortn;Table Mtn. Line 1 T-1947 W DEPTH 2-WAY TIME (FT) (SEC) (ntm Ufl) DATU M = / M\\ t M 1 I ! ' t o p Kp iwsv*. C.^'t $K 5 ^Psi — 3 o o a - ,*“ ;* .•*> > s >/ '-.. s' V W \ . 1. Ifc W ** .“H*. ■>l\.fr T Z 2 . **r +*0.1»^q.* to'.'<*'>£ Mb > S ■J-IrLJrxf V^vV ;V( K P*t r ••'tw-ji •*-» • —1 >;>cx• - '. .*• I u^r.t-Vc -'.1^01^ f, '►V. .»BW« ».!► * b * l O 5 0 0 — » ‘+< y '* •a• *" p < f i:f <.> is >i I , P€ ■:•> V; v yf', KV -' "h VJ‘ <> , >y :‘„j, t- »'>;. _» |i ';,. •,, /• <•; J ._ '■ n*- N ■ 'n*V '■ Figure 21. Interpreted final section ®f North Table Mtn. Line 2 T-1947 35 TABLE 1 Calculated interval and average velocities, from Money (1977)- Well Interval Average Depth Elevation Thickness Velocity Velocity Formation (ft.) (ft.) (ft.) (FPS) (FPS) Laramie 190 + 5363 970 Fox Hills 1160 +4393 90 7,500 7,500 Pierre 1250 + 4303 1280 8,000 7,700 TZ1 2530 +3023 1250 TZ2 3780 +1773 11,000 9,400 1610 Terry Sand 5390 + 163 1000 9,500 9,400 Hygiene Sand 6390 - 837 2110 10,000 9,500 Niobrara 8500 -2947 - 290 Benton 8790 -3237 380 Dakota 9170 -3622 11,500 10,000 280 Morrison 9450 -3897 200 Lykins 9650 -4097 609 Lyons 10259 -4706 T-1947 W a.3 Zt ±4, XO 8 7 C 5 4 3 2 1 VP # 83oo - i — I I I l ( i i I I I 36 8 8 0 0 - - 8 o o o - - undi £f«rerrfi a+«d \ i Top k p Psx " ^Tri <-TIl Figure 22. I Geologic section SL -- Se. Table Mtn< p-t (VE: 0> K Kn k b undiffercn^i ATcd “■5000 (c-f. Fig. 13) p€ SSW T“1947 NWE 37 und i fi>cren+ia4'«d Psi TZi. TZZ Figure 25* SL Geologic section Ne. Table Mtn, Line 1 Kb undifferenh' aiccl (c-f. r,j.i3) T-1947 (NTH (LMFlV WNW ESE 5 4 >500 i 3 2 L ve * i -J » * 38 7 300 Uftdi a.+ *d 5 000, Top Kp KPs i ^ T Z l TZZ Figure 24. SL-- Geologic section No. Table Mtn* A Line 2 (VEs 0) V Kn K b und i ffe re«4*/ a^ed 5 0 0 0 - - Ccf. F ij.1 3 ) p c T-1947 39 i- ol CC c HS _ eo y M r* m t-* 0- «l >4 . C V- W C v v c a. c o ”« u o o »T> 04 w QT>3 T-1947 Interpretative conclusions resulting from study of the seismic data are concisely enumerated below and then dis cussed in detail in the next section. A. Seismic Observations 1. The final seismic time sections exhibit a high frequency character, which is apparently the result of these factors: vibrating on a high velocity (rigid) material, the enhancing of approximately 34 Hz energy by constructive interference in the lava, and low-cut filtering used in processing. 2. A very high amplitude surface wave was recorded on South Table Mountain. The ground roll recorded on North Table Mountain was of only slightly lower amplitude as a result of the longer spread geometry. 3. Although a very high longitudinal wave velocity was not observed for the mafic latite, an appropri ate 10,000 feet-per-second shear wave (direct arrival) velocity was noted. The effect of the caprock was also evident in its creating numerous small static offsets. B. Geologic Observations 1. The Denver basin sedimentary horizons are low- * dipping under the Table Mountains. A small south east dip component (less than two degrees) is observed. 41 T-1947 2. No igneous source vent is evident under the portions of North Table Mountain surveyed, including the conical knolls location. 3. Two faults not previously reported are present under the Table Mountains. 4. The South Table Mountain fault has a vertical plane projecting from the Precambrian basement through the Pierre Formation. The eastern block dropped down relative to the western, and displacement of geologic horizons across the fault is essentially uniform. 5. The North Table Mountain fault is also vertical, extending from the basement through the Pierre Formation, and its eastern block is down-dropped. However, the displacement of geologic strata across the fault plane tends to increase with depth, indicating penecontemporaneous fault movement in volving the basement, associated with uppermost Cretaceous sedimentation. 6. The North Table Mountain fault is the apparent southward continuation of one of the basin-margin faults reported by Money (1977, p.37). [ARTHUR CAKES EIBRARY, COLORADO SCHOOL of MINES GOLDEN, COLORADO BQ4QS 42 T-1947 Discussion The character of the seismic data recorded in the study is similar to that obtained by Shuck (1976) and Money (1977), using the same equipment over sedimentary -- rather than igneous — terrain. The very high (compressional wave) velocity anticipated in the igneous medium is not observed. However, as noted earlier, a 10,000 feet-per-second first (direct?) arrival is apparent on the section in Figure 6. This velocity corresponds to shear wave speeds in basalt tabulated by Press (1966, p.198). Seismic energy was effec tively transmitted and recorded through the mafic latite without abnormally high attenuation. In fact, the frequen cies recorded here are slightly higher than those obtained by Shuck and Money. The rigid caprock appears to have en hanced the high frequency spectrum of the seismic pulse. Furthermore, the thickness of this surface layer is roughly equivalent to h wavelength for 34 Hz energy (Fig. 3), which was possibly intensified by constructive interference. The less than 200 feet composite thickness of latite caprock was clearly no obstacle to the seismic pulse of greater than 3 00 foot wavelength. A high-amplitude surface wave (about 18 Hz frequency, / o o ft. wave-length, 5400 ft./sec. velocity) was recorded on j a. 11 three lines, in spite of the 300 foot geophone spacing ARTHUR LAKES LIBRARY COLORADO SCHOOL of MINES GOLDEN, COLORADO 8Q4QI 43 T-1947 used on North Table Mountain. To eliminate this noise from the reflection sections, a combination of low-cut filtering and muting was applied in computer processing. The resulting final sections have a high frequency appearance. The sediments of the Denver basin, located beneath the Table Mountains, were previously known to dip slightly toward the southeast (e.g., Stewart, 1953, Plate 10). Authors such as Johnson (1934, p.21-2) and Ahmad (1971, p.88) hypothesize the existence of an igneous vent beneath North Table Mountain, from which the four flows of latite capping the Table Mountains were assumed to have been extruded. Line two of this survey passed directly over the conical knolls section of North Table Mountain, suspected by Ahmad to overlie the remnant source vent (Fig. 18). No such sub surface igneous structure is observed on the seismic sections acquired. The nature and extent of faulting in the area surround ing the Table Mountains has been discussed earlier in the paper. Prior to this investigation, geologic studies of the Table Mountains have detailed a single fault located on the Clear Creek side of South Table Mountain (SW%, Sec. 26, T.3S., R.70W.): Waldschmidt (1939, p.41), Ahmad (1971, p.33). The location of this fault is depicted clearly by Van Horn (1957, 197 2) on his maps of the Bedrock Geology of the Golden Quadrangle. Van Horn (1957) describes the throw and heave on this fault to be about 6 0 and 4 0 feet, 44 T-194 7 respectively. The eastern hanging wall is displaced to the west over the footwall, making it a high-angle reverse-type fault. The vertical fault under South Table Mountain revealed by this study does not penetrate the surface. However, the projection of its fault plane cuts the surface about 900 feet east of the high-angle reverse fault mapped by Van Horn. The eastern block of the vertical fault has been uniformly dis placed downward about 60 feet, as measured at each of the reflectors down to the Niobrara (Fig. 26). The offset of reflectors below the Niobrara may be somewhat greater than 6 0 feet. This fault displacement pattern suggests that 60 feet of movement occurred during a single (uppermost Creta ceous ?) event, perhaps along a plane of previous faulting (i.e. prior to upper Cretaceous deposition). The proximity and same magnitude displacement of the. high-angle reverse fault and the vertical fault suggest a possible subsurface intersection (Fig. 22). However, neither the reverse fault nor an intersection of the two faults can be seen on the South Table Mountain seismic section in the low redundancy data (resulting from muting) above 0.4 seconds. Furthermore, there is no obvious fault mechanism to link the two different styles of faulting. Because the South Table Mountain fault does not appear in the seismic sections obtained on North Table Mountain, 45 T-1947 its trend — assuming it does not die out rapidly to the north — is probably northeastward. Davis and Weimer (1976, p.283) report dominantly northeastward trending faults in the Boulder-Weld zone north of this study area. Money (1977, p.40) describes a fault offset at Niobrara level in seismic data covering NW%, Sec. 18, T.3S., R .6 9W. A north eastward projection of the South Table Mountain fault, roughly parallel to the strike given by Van Horn (1957, 197 2) for the reverse fault, could intersect the location given by Money (Fig. 25). Surface geologic studies of North Table Mountain have not revealed the existence of the vertical fault appearing on the seismic sections acquired there. This fault apparently extends from the basement through the Pierre Formation and dies out beneath the extrusive Paleocene surface. Figure 26 is a graph of the amount of displacement of each geologic reflector across this fault. It should be compared with similar profiles obtained by Davis and Weimer (1976, p.296) and Money (1977, p.38). The shape of the curve obtained here is like that calculated by the others, and the magnitude of displacement is very much the same as that given by Money for basin-margin fault four. The similarity in character (vertical, basement-controlled, eastern block down-dropped) and displacement profile, and the close proximity of the North Table Mountain fault to Money's T-1947 46 noftxzou eol ogic 47 T-1947 basin-margin fault four strongly suggest these faults are one and the same. Figure 25 depicts the interpretation that these two faults are continuous. Money (1977, p.41-50) discusses the growth dynamics on this basin-margin fault. The displacement curve (Fig. 26) for this fault implies movement initiated by basement uplift about the time of Pierre-Terry deposition. Subsequent inter related mechanisms of basement tectonics and the rapid, un stable sedimentation reported by Weimer (197 3, p.70) account .for growth along the fault. It is important to emphasize here that the basin-margin fault under North Table Mountain is not a typical listric normal growth fault. In his paper which first suggests the presence of the basin-margin-type fault in this locality, Weimer (1973, p.57) maps such a fault with several well-documented control points of observation. Subsequent work by Shuck (197 6) and Money (1977), and Davis and Weimer (1976) indicates the existence of a zone of basin-margin-type faults, of dominantly north-south trend (Fig. 27). Alternate Interpretations Although arguments have been advanced in favor of inter pretations adopted in this study, certain contrary or alterna tive hypotheses have not been disproved. T-1947 o t* v>Ui QC -J3 O— Ui Vt (IIat 49 T-1947 Because of the vertical character of the faults dis covered and the lack of interval thickness differences across the fault planes, strike-slip or wrench faults might be interpreted, and a single-event movement might be argued. Interpretation pitfalls of this variety are discussed by Tucker and Yorston (1972, p. 12-13). The continuity of Money's basin-margin fault four with the North Table Mountain fault appears highly probable but could be disputed for lack of intervening control. The projection of the South Table Mountain fault has an even greater uncertainty and is therefore hypothesized rather than asserted by this author. The identification of the North Table Mountain fault on the extreme southern end of the north-south line is questionable. The fall-off in redundancy (fold) of data near the end of the survey line, combined with uncertain acoustic "edge-effects" close to the mountain edge, render such interpretation risky. The existence and location of this fault, therefore, is based primarily on the data of the east-west line. It seems unlikely, but is not disproved, that an igneous source vent might indeed exist beneath North Table Mountain in a location not covered by this survey. T-1947 The contrary interpretations just noted are acknowledged as possible alternatives to the conclusions listed :in this study. However their validity is rejected on the strength of the rationale advanced in support of the interpretations adopted. m T H U R EAKES LIBRARY C O L O R A D O S C H O O L of M I N * S GOLDEN, COLORADO 80401. 51 T-1947 CONCLUSIONS The conclusions from this investigation are clearly evident from the foregoing but are reiterated here for con venience : 1. The seismic field parameters (i.e. source, spread geometry, signal frequency, etc.) resulted in the acquisition of high-quality reflection sections. Sufficient seismic energy penetrated the relatively thin (less than 2 00 foot composite thickness) igneous surface layer to provide data to the Precambrian basement (greater than 12,000 feet). 2. The seismic sections obtained document the gentle southeast dip of Denver basin strata in this location. 3. The seismic evidence does not support the North Table Mountain source vent hypothesis. 4. Two significant Laramide age, basement-controlled vertical faults are detailed. One is the southward continua tion of a basin-margin fault previously mapped. Its movement history and dynamics are complex. The other is a basement- controlled vertical fault which apparently moved in a singular episode. 52 T-1947 REFERENCES Ahmad, Mahmood U. , 1971, Study of the genetic relationship between the Ralston intrusive bodies and the Table Mountain lava flows near Golden, Colorado (M.S. thesis): Golden, Colorado School of Mines, 124 p. Davis, T.L., 1974, Seismic investigation of Late Cretaceous faulting along the east flank of the central Front Range, Colorado (Ph.D. thesis): Golden, Colorado School of Mines, 65 p. Davis, T.L., and Weimer, R.J., 1976, Late Cretaceous growth faulting, Denver basin, Colorado: in Professional Contributions of the Colorado School of Mines - Studies in Colorado Field Geology, no. 8, p. 280-300. Dobrin, Milton B., 1960, Introduction to geophysical prospect ing: New York, McGraw-Hill, 446 p. Emmons, S.F., Cross, W. , and Eldridge, G.H., 1896, Geology of the Denver basin in Colorado: U.S. Geol. Survey Mon. 27, 556 p. Griffits, M.O., 1959, Zones of the Pierre of Colorado: Am. Assoc. Petrol. Geologists Bull., v, 33, p. 2011-2028, Johnson, J.H., 1925, The geology of the Golden area, Colorado: Colorado Sch. Mines Quart., v. 20, no, 3, 25 p. ______, 193 0, The geology of the Golden area, Colorado: Colorado Sch. Mines Quart., v. 25, no. 3, 33 p. ______t 1^34, Introduction to the geology of the Golden area, Colorado: Colorado Sch. Mines Quart., v. 29, no. 4, 36 p. LeRoy, L.W. and Weimer, R.J., 1971, Geology of the 1-70 road cut, Jefferson County, Colorado: Colorado School of Mines Prof. Contrib. No. 7. Lovering, T.S., and Goddard, E.N., 1938, Laramide igneous sequence and differentiation in the Front Range, Colorado: Geol. Soc, America Bull., v. 49, p. 35—68. 53 T-194 7 Money, Nancy R. , 1977, A seismic investigation of the north Golden area, Jefferson County, Colorado (M.S. thesis): Golden, Colorado School of Mines, 56 p. 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Shuck, Edward L., 1976, A seismic survey of the Ralston area, Jefferson County, Colorado (M.S. thesis): Golden, Colorado School of Mines, 45 p. Stewart, W.A., 1953, Structure and oil possibilities of the west flank of the Denver basin, north-central Colorado (Ph.D. thesis): Golden, Colorado School of Mines, 121 p. Tucker, P.M., and Yorston, H.J., 1973, Pitfalls in seismic interpretation: Soc. Exploration Geophysicists, Monograph 2, 50 p. Van Horn, R., 1957, Bedrock geology of the Golden quadrangle, Colorado: U.S. Geol. Survey Geol. Quad. Map GQ-103. ______, 1972, Surficial and bedrock geologic map of the Golden quadrangle, Jefferson County, Colorado: U.S. Geol. Survey Geol. Quad. Map I-761-A. 54 T-1947 Waldschmidt, W.A. , 193 9, Table Mountain lavas and associated igneous rocks near Golden, Colorado: Colorado Sch. Mines Quart., v. 34, no. 3, 62 p. Weimer, R.J., 1973, A guide to Uppermost Cretaceous strati graphy, Central Front Range, Colorado: deltaic sedi mentation, growth faulting and early Laramide crustal movement: Mtn. Geologist, v. 10, ho. 3, p. 53-97. ______, 1976, Cretaceous stratigraphy, tectonics and energy resources, western Denver basin: in Pro fessional Contributions of the Colorado School of Mines - Studies in Colorado Field Geology, no. 8, p. 180-227. Weimer, R.J., and Land, C.B., Jr., 1975, Maestrichtian deltaic and interdeltaic sedimentation in the Rocky Mountain region of the United States: The Geol. Assoc. Canada Sp. Paper 13, p. 633-666.