I e\nt~n voo. 8. th* LATE CRETACEOUS GROWTH FAULTING, DENVER BASIN, COLORADO T. L. Davis Colorado School of Mines Golden, Colorado 80401

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1 I e\nt~n voo LATE CRETACEOUS GROWTH FAULTING, DENVER BASIN, COLORADO T. L. Davis Colorado School of Mines Golden, Colorado R. J. Weimer Colorado School of Mines Golden, Colorado Abstract Interpretation of 250 mi (413.3 km) of reflection seismic in conjunction with surface maps and well data along the east flank of the Denver basin reveals two distinct types of Late Cretaceous faulting. An early Laramide, basement-controlled fault system is the dominant structural style in the zone of flank deformation. Basinward from the basement-controlled fault system, an associated new tectonic style has now been recognized in the Cretaceous foreland basin. Deltaic sedimentation and overpressured shale masses initiated a shallow depth growth fault system similar to the tectonic style of many Cenozoic sequences along continental margins. The shallow growth fault system is approximately 10 mi (16.6 km) wide and 30 mi (50 km) long and affects uppermost Cretaceous strata. The seismic data indicate three or four major trends of listric normal faults that do not appear to extend below a depth of 5,000 ft (1,530 m) in the Pierre Shale. Antithetic horst-graben faults are found on the basinward side of each major fault. Near surface, growth fault movement is indicated by a five-fold thickening of the Fox Hills Sandstone from a normal 75 ft (22.8 m) to 400 ft (122.4 m), and the presence of thicker mineable coal beds in the Laramie Formation in downthrown blocks. Recognition of growth fault systems will play an important role in future exploration for petroleum and coal in the Rocky region. Mountain Introduction In mineral exploration, geologists and geophysicists search for anomalies, i.e. areas where geologic conditions differ from the norm. Some anomalies may contain minerals that can be produced at a profit; understanding of these anomalies may pave the way for new thinking in exploration. New thinking means new prospects. This approach is the key to future mineral exploration. It may be used in analyzing old, well-known areas or in exploring new areas. This paper describes an area on the west flank of the Denver basin, Colorado, that contains several geologic and geophysical anomalies. The area is immediately east of the Front Range uplift in portions of Boulder and Weld counties (fig- D- Normally, all Late Cretaceous faulting in the Rocky Mountain region is considered a* basement-controlled. A fault zone, mappf d at the surface, is extended to the and related to deformation during the mide orogeny after the sediments were de posited. The geologic and geophysical anomal»f in this study cannot be explained by * e traditional concepts. The observat th* data document a new tectonic style Cretaceous foreland basin. The style - shallow-growth faulting, associated deltaic sedimentation; its recognitj n relevant to exploration for petroleum coal in the Rocky Mountain region. ^

2 R 7 1 W ' ". - "Figure 1. Tectonic and geologic map of study area. Seismic faults are shown by heavy lines; light lines are major surface faults or faults found in mines. B indicates basement faults. Symbols are shown on figure

3 The west flank of the Denver basin has had a long history of sustained exploration and production of petroleum and coal. Petroleum production is from both structural and stratigraphic traps. The pay intervals are the upper Dakota Group and the Terry and Hygiene Sandstones of the middle Pierre (Hygiene Zone) (fig. 2). The oldest field in the Denver basin is the Boulder oil field (fig. 1), discovered in 1902 (Fenneman 1905). It has produced oil from fractured Pierre Shale (Hygiene Sandstone). Recent discovery and development of the Wattenberg Field (fig. 1), part of which extends into the eastern part of the study area, has renewed exploration interest in the area (see Matuszczak, paper 22, this volume). The Wattenberg field produces gas from the J Sandstone of the Dakota Group at depths of 8,000-9,000 ft (2,448-2,754 m). During development of the Wattenberg field, oil production was established from the Terry and Hygiene Sandstones of the Pierre Shale (fig. 2) in Spindle and nearby fields (see Moredock and Williams, paper 21, this volume). Producing depths are 4,500-5,000 ft (1,377-1,530 m). Coal production from the Boulder-Weld coal field was begun before Three main coal beds were mined from the lower 125 ft (38.1 m) of the Laramie Formation. Spencer (1961) reported coal bed thickness ranging from a wedge-edge up to 40 ft (12 m). Exploration and production of energy resources continues today, and the geologic and geophysical concepts presented herein should aid in future programs. Geology of West Flank, Denver Basin Structure - The area of investigation is located on the west flank of the Denver basin (fig. 1). The regional geologic setting of the area can be described in terms of two crustal blocks. The upthrown Front Range block, exposing a Precambrian metamorphic and igneous complex, flanks the western portion of the area. The downthrown Denver basin block is an asymmetric structural basin that contains up to 13,000 ft (3,978 m) of 3 3 7> ^ D O 3 t D \J I AGE TERT. ETACEOUS CC (j UPPER LOWER CRET. Jurassic TRI. PERM.- PENN. PREC. (Kp) Transition < UJ v o- u. o o </> Ul o N * 3K V) W /S&? ^-'o o a- T^o^s I^^.~ 0 0 0,0 o o o _-^-L_ - ^o r i o<> ~~ ~~ ~ J o o O O O O rt-ltt ~ -I-I-I-I-.1. -L 1_ _.!_ HZ~i:r[:~" o o o * J O > J O o~o JL.I o o o o o o o o 0 0 o.. o o o To^Ti, / / + "*" +\s FORMATION Laramie. \ Klf Fox Hllls^ MKR 1 (TZ1) MKR 2CTZ2) Larimer- Rocky Ridge Terry Hygiene Niobrara Kn Benton Group Kb Dakota Group Kd P Morrison Lykins Lyons a Fountain pcgs Jm Figure 2. Generalized diagram showing t n», nations and seismic reflection horiz west flank Denver basin (not to sea 282

4 sedimentary section ranging in age from Paleozoic to the present. The Upper Cretaceous, up to 9,000 ft (2,754 m) thick, dominates the sedimentary section (fig. 2). In a 3 to 6 mi (5-10 km) wide zone between the 2 blocks, the sedimentary sequence is deformed into an east-dipping monocline cut by high angle basement faults (figs. 1 and 3). Dip of the strata varies from 15 to overturned. Stratigraphic section may be cut out by faulting in this zone of deformation. One of the largest structural anomalies in the Denver basin is the horst-graben faulting in T. IS., Rs W., and Ts. 1 and 2 N., Rs. 67, 68, 69 and 70 W. (fig. 1). Much of the information about individual faults is from coal mines within the fault zone. Colton and Lowrie (1973) compiled the mine data in an area they referred to as the Boulder-Weld coal field. In this paper, the overall area of faulting is referred to as the Boulder-Weld fault zone. The regional structural dip in the area is 2-4 to the southeast; the structural strike is northeast. This gentle flank of the Denver basin is broken by the fault zone which is 10 mi (16.6 km) wide, 30 mi (50 km) long, and consists of numerous branching "en-echelon" faults with near-surface displacements from a few ft up to 500 ft (469 m). Although the overall fault zone is northeast-trending, individual faults vary from dominant northeast trends to minor northnorthwest trends (fig. 1). The structural style is one of horst-graben fault blocks that vary from one-quarter to 2 mi (379 m- 3.3 km) wide and several miles long. The fault planes are not well exposed, but, where observed, they are high angle and either reverse or normal. One of the best descriptions of the faulting is by Spencer (1961) covering the Louisville Quadrangle in the southwest portion of the fault zone. Strata within the fault blocks are warped into anticlines and synclines. The larger anticlines have been drilled as prospects for petroleum, but none has been productive. Stratigraphy - The Boulder-Weld fault zone is identified by offsets mainly in the upper Pierre, Fox Hills and Laramie Formations (figs. 1 and 2). The fault zone contains three Stratigraphic anomalies: (1) Unusual thicknesses of the Fox Hills Sandstone. (2) Mineable thickness of coal in the graben area compared to thinner coal beds over the horst areas. (3) High rates of sedimentation in the uppermost Cretaceous sequence. WEST 10,000' -I S.L- -10,000' VERT. EXAG. X2 -I "igure 3. Geologic cross section showing surface faults in Boulder-Weld fault rone extending..'.;' to Precambrian (modified after Haun 1968). Location approximately along A-A 1 (fig. 1). 283

5 For the western Denver basin, the depositional model for the Pierre, Fox Hills and Laramie has been reconstructed by Weimer (1973) as a regressive deltaic sequence (fig. 4). The Pierre Shale is a shelf-prodelta deposit of thick shales with lesser amounts of siltstone and minor sandstone. The Fox Hills Sandstone is fine- to medium-grained sandstone interpreted as shallow marine in origin, either as a delta front or beach and shoreface. The coal-bearing Laramie Formation is sandstone, siltstone and clay of delta plain origin. The environments of deposition are largely fresh water, although oysters and thin, burrowed beds suggest minor incursions of brackish to marine water. ^ ^ PROGRADATION DELTA PLAIN DELTA FRONT (LARAMIE -ii. tilt., clay) (FOX HILLS -»») PRODELTA (PIERRE SM) Figure 4. Delta sedimentation model relating formations to facies to environments of deposition (after Weimer 1973). In this study, the first stratigraphic anomaly was observed in the White Rocks area (sec. 18, T. IN., R. 69 W.). surface exposures (fig. S) indicate two regressive cycles of Fox Hills Excellent Sandstone (Weimer 1973, p ) with a total thickness of approximately 160 ft (48.9 m). section was measured on both sides of a north-trending high-angle normal fault The (fig. 5). A hole was drilled by the Colorado School of Mines Geophysics Department as a part of a research project in interpreting depositional environments from borehole measurements (Bedwell 1974). The hole was cored continuously to a depth of 343 ft (105 m) and then drilled to total depth of 390 ft (119.1 m). Core descriptions were reported by Weimer (1973) ; environmental interpretations are summarized on figure 6. Mechanical logs for the core hole are shown on figure 7. cycles of regressive, dominantly shallow Four Figure 5. Aerial view of Fox Hills Sandstone at White Rocks (sec. 18, T. 1 N., R. 69 W.). Arrow points to location of CSM core hole shown on figure 6. F denotes fault; D is downthrown block. View to north. water, marine sandstone are clearly delineated in the cores and logs. Cycles vary in thickness from ft ( m). According to surface mapping by Trimble (1975), the top of cycle 4 is thought to be at least 70 ft (21.3 m) above the ground elevation of the core hole. While a normal Fox Hills section in the western Denver basin has one or two sedimentation cycles, varying from ft ( m), the anomalous Fox Hills section at White Rocks contains four stacked cycles whose total thickness is estimated to be 430 ft (131.4 m). Weimer (1973) advanced the theory that local growth faulting could best explain the Fox Hills section at White Rocks. Elsewhere in the Boulder-Weld fault zone, exceptional thicknesses for the Fox Hills in the Louisville Quadrangle were reported by Spencer (1961) and Rahmanian (1975). From surface measurements and drill hole data, the Fox Hills varies in thickness from ft ( m). In an 8 sq mi (13.3 km ) area southeast of Boulder in T. IS., R. 70 W., Rahmanian (1975) determined the Fox Hills was thicker in major graben areas than in the area of horst blocks. These data are similar to White Rocks and also suggest fault movement in the Boulder-Weld fault zone at the time of Fox Hills deposition. The second stratigraphic anomaly in tne area is thickness variation of coals in the 284-

6 r 5400' X ' FOX HILLS S.S. FOX HILLS 5200' L 5100' B.F. Figure 6. East-west structural cross section through White Rocks area. Location is indicated on figure 1. S.F. is near surface expression of a seismic fault. B.F. is seismic basement fault. Four sedimentation cycles in Fox Hills Formation are shown by CSM core hole. DF = delta front; C = channel fill; CO = coal; S = shoreface; B = beach; PD = prodelta; T = marks transgression. In the Louisville Quadrangle, Spencer (1961) reported mining of sub-bituminous B coal from three main beds. The map compiled by Colton and Lowrie (1973) indicates that the mines in the Boulder-Weld coal field are in an area 6 mi (10 km) wide and 25 mi (41.6 km) long, generally corresponding with the southeast margin of the overall fault zone (fig. 1). The majority of the old abandoned, underground coal mines are clearly in graben blocks of the fault system. located lower 125 ft (38.1 m) of the Laramie Formation. Mineable thickness of coal is generally thought to have been 4 or 5 ft ( m). Although present on the horst blocks, coal is much thinner here and, therefore, was not mined. Like the underlying Fox Hills, thicker coals in the graben fault blocks, compared to the horst blocks, indicate penecontemporaneous (growth) fault movement. The faults were active during deposition of peat in freshwater swamps. A greater thickness of peat accumulated on the downthrown side of faults compared to the upthrown side. The third stratigraphic anomaly relates to high rates of sedimentation for the uppermost Cretaceous. The 7,500-8,000 ft (2,295-2,448 m) section of Pierre Shale in the Golden-Boulder area has long been recognized as the thickest Pierre section in the Denver basin. Scott and Cobban (1965) described the Pierre Shale in the Front Range area and mapped the distribution of faunal zones. Recently, Obradovich and Cobban (1975, p. 36) have published potassium-argon radiometric dates for bentonite beds associated with faunal zones in the Late Cretaceous. By ammonite correlations

7 CSM LOG -o' zoic deltas throughout the world. These three stratigraphic anomalies suggest a genetic relationship between high rates of deltaic sedimentation and growth faulting in the Boulder-Weld fault zone. Figure 7. Mechanical logs from CSM core hole for Fox Hills Sandstone. GR - gamma ray; SP «spontaneous potential; R = resistivity; N = neutron; (compiled from Bedwell 1974). Four sedimentation cycles determined from cores are indicated. in the Pierre Shale, these data suggest that the upper 4,000 ft (1,224 m) of the Pierre Shale (Transition Zone, fig. 2) was deposited in a 1-2 million-year interval dating from approximately m.y. Even though these dates may be subject to revision, the rates of sedimentation are high when compared to other parts of the upper Cretaceous. The overlying Fox Hills and lower Laramie, having a combined thickness of 1,000-1,500 ft ( m) probably had similar high rates of sedimentation. In the Tertiary of the Gulf Coast, Curtis (1970) and Bruce (1973) report growth faults associated with deltaic depocenters. Growth faulting occurs in high constructional Ceno- Seismic Investigation Structure - An important aspect of the Boulder-Weld fault zone anomaly is whether or not the faults extend to the basement as suggested by earlier workers (e.g. Spencer 1961, Haun 1968). To resolve the question, a seismic survey of the area was undertaken by Davis (1974). Most geologists would draw a typical cross-section across the faulted area shown on figure 3. All faulting would be assumed to be associated with the Laramide orogeny comprising basementcontrolled, block faulting and associated horst-graben systems. Interpretation of a regional seismic line (fig. 8) shows that basement-controlled fault systems exist only in the north and west extremity of the area. The seismic data, however, do not appear to support a basement-controlled tectonic style for the fault system in the central portion of the area. Persistent reflections occur from both the Niobrara and Hygiene horizons. At these levels no faulting can be detected underlying the prominent surface fault zone but subtle anticlinal features appear at the Hygiene level and below. Thus, the seismic data reveal that two distinct fault systems are present in the area. The deep, basementcontrolled fault system occurs mainly within the zone of flank deformation with near vertical dip on the fault planes. A shallow, listric, normal fault system extends basinward from the edge of the deep fault system. Three of four major trends of listric normal faults, mapped by seismic methods within the Boulder-Weld fault zone, are shown by heavy lines on figure 1. Fault planes have high dips near the surface which diminish with depth, passing into bedding planes. Fault planes dip to the east and blocks are downthrown in that direction. These faults extend vertically for approxi- I hi 286

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9 mately 5,000 ft (1,530 m), dying out somewhere in or above the Pierre Hygiene Zone. Seismic evidence suggests that antithetic faults occur on the basinward side of many of the major shallow faults. As suggested by the surface fault pattern, however, there are numerous antithetic faults with displacements too small to be interpreted from seismic. Based on seismic information and well-control, a generalized regional geologic cross section would depict the structure as shown on figure 9. Interpretation of 250 miles of seismic data, taken throughout the area (fig. 10), was made easier by reprocessing the data to enhance the shallow data and integrate seismic and well log information to establish adequate subsurface velocity control. Because of the scarcity of velocity information from lack of sonic logs or checkshot velocity surveys in the area, a "bootstrap" method proved useful in obtaining velocity information from induction logs. Such a scheme is amenable to areas of uniform lithology in which velocity information is limited, relative to conductivity (or resistivity) data. Using only those wells which had both sonic and induction logs available, an empirical logarithmic relation between conductivity and velocity NW 9-IN-70W IO-IN-70W A 1SE S.L. --4OOO Figure 9. Composite structural cross section reconstructed from regional seismic section a nd well data. 288

10 PIERRE SHALE VELOCITY-CON DUCTIVITr RELATION 200 CONDUCTIVITY (M Figure 10. Heavy lines show location of 250 mi (416.6 km) of seismic lines used in this study. Rl, R2, R3 and R4 mark location of composite regional seismic line (fig- 8). (reciprocal of transit time) was derived by crossplotting techniques (fig. 11). Interval velocities were derived in wells which had only induction logs run and were used to supplement the existing velocity control. Synthetic seismograms were generated from both sonic and induction log data (fig. 12), enabling determination of the nature of reflected seismic events and their spatial location in the subsurface. This velocity information, in conjunction with some seismic velocity analysis data, made it possible to prepare structure contour maps on several horizons (figs ). The structure contour maps illustrate the extent of the basement-controlled fault system area within the zone of flank deformation in the west and northwest portion of the area. The greatest displacement on this fault system occurs in the extreme west por- tion. The axis of the Denver basin is reflected by the reversal in strike in the south-central map area at the Precambrian and Niobrara levels. A fault system is not apparent at these levels in the central map area. An arcuate fault system can be seen within the Transition Zone at TZ1 and TZ2 levels. Displacements on this shallow fault system are greatest in the western portion of the area as well. Growth Faulting - A possible clue to the nature of the fault systems present along the west flank of the Denver basin is the concept of fault displacement. Figure 11. Velocity-conductivity crossplot. Displacement refers to the relative movement of two sides of a fault. A measure of displacement is the relative offset of a marker horizon across the fault. If the displacement is the same for every marker horizon then the fault can be considered as occurring in a "single event". However, if the displacement differs across the fault from 289

11 seismic trace Figure 12. Synthtttc Synttwtk *>»> ****** Log Fro* SOT* u «- Kh> II 5 S 6HN-69W Velocity Loq -*- TZ2 Synthetic seismograms. marker-horizon to marker-horizon, then the fault can be interpreted as having undergone recurrent movement and growth. Relative displacement thus provides a means of analyzing time and rate of fault movement. A plot of relative fault displacement of several markerhorizons, based on both seismic and well information (fig. 18) reveals: 1. Displacement (thickness) changes of several hundred feet occurring basinward across the deep, basement-controlled fault system (T. 2 N., R. 69 W.) in the Uppermost Cretaceous (Post-Terry). This fault movement may mark the beginning of Laramide orogeny Khy along the east flank of the central Front Range. Recorded displacement changes and recurrent fault movement extend from upper Hygiene Zone throughout the Uppermost Cretaceous. 2. Displacement on the major shallow listric faults increasing with depth. Thic.kness changes of several hundred feet occur across these faults at least throughout post-tz2 time. These faults die out into bedding plane faults in the lower Transition Zone or upper Hygiene Zone, thus establishing the initial development of these faults as occurring during a corresponding time interval. Growth of these features continued with subsequent Upper Cretaceous sedimentation. 3. Similarity of the displacement curves for the deep and shallow fault systems revealing that both fault systems recorded recurrent movement and growth during the Late Cretaceous even though the nature of the faulting differs. Overpressured Shale Masses - An isopach map between the TZ1 and TZ2 marker horizons is shown in figure 19. This interval encompasses the most uniform shale interval of the Transition Zone. The isopach map documents substantial thickening of this interval on the downthrown sides of the major shallow growth fault systems. It also depicts a local TZ1-TZ2 isopach in the central map area (fig. 19). This localized thickness coincides with a zone of low TZ1- TZ2 interval velocity (fig. 20). Mechanisms thought to account for these anomalous features include: formation of a local depocenter during Transition Zone deposition and, generation of a low-density, overpressured shale swell. Examination of sonic and induction logs from two wells within the anomalous region reveal abrupt trend reversals in the TZ1-TZ2 interval. These trend reversals could be caused by fluid pressure gradients of 0.2 to 0.3 psi' ft greater than normal. Seismic evidence for abnormal pressure in the Pierre Transition Zone may appear» figure 8. Abrupt isochron expansion wit" 290

12 Contour Interval = 400' Figure 13. Top of Precambrian structure contour map from seismic data. 291

13 R 71 W Contour Interval = 400 Figure 14. Top of Niobrara structure contour map from seismic data. 292

14 Contour Interval = 400 Figure 15. Top of Hygiene structure contour map from seismic data..:' 293

15 R 71 W Contour Interval = 100 Figure 16. Structure contour map of Transition Zone (marker 2) of Pierre Shale- 294.

16 R 71 W Contour Interval = 100 Figure 17. Structure contour map of Transition Zone (marker 1) of Pierre Shale.

17 O DEEP FAULT GEOLOGIC MARKER Figure 18. Fault displacement of various geologic markers. the Transition Zone is evident on the "upthrown" side of many of the major shallow growth faults. These isochron anomalies may be associated with transmission of seismic waves through a high-pressure shale swell in the Transition Zone and the concept that "flowage is slowage" (Tucker and Yorston 1973, p. 17). The Hygiene and reflecting horizons below are "pushed down", by 30 to 40 msec from regional, below these Transition Zone isochron anomalies. The subtle anticlinal features, at the Hygiene and Dakota level on seismic sections, can be entirely created by the overlying combination of overpressured shale masses and growth faults in the Upper Pierre Transition Zone (fig. 21). A New Rocky Mountain Tectonic Style Basement-controlled fault systems have generally been regarded as the dominant central Rocky Mountain structural style. An associated, new tectonic style must be invoked in the Denver basin to explain the nature of the shallow fault system as outlined from seismic data. A nodel, following concepts summarized by Curtis (1970) and Bruce (1973), designed to explain the nature of the shallow fault system, involves formation of a regional growth fault system through the processes of tectonism and deltaic sedimentation. Successive phases of the model as depicted in figure 22 include: 1. A deltaic depocenter was formed by tectonism related to uplift of the Front Range. The depocenter formed on the basinward side of the deep, basement-controlled fault system during deposition of the lower Transition Zone. control, the depocenter Once established by fault localized sedimentation throughout the remainder of the Late Cretaceous. 2. A large thickness of sediment was recycled into the Pierre seaway along the east flank of the uplift area. Extremely high rates of sedimentation occurred in depocenter systems along the flank of the uplift. It is estimated that the entire Transition Zone of the Pierre shale was deposited in 1-2 million years. Prograding sequences of deltaic sediments derived from the Front Range uplift continued to infill the depocenter. Sediment loading, subsidence, and recurrent movement on the deep fault system resulted in the generation of high fluid pressures and the initiation of growth faulting. 3. Continued progradation of deltaic sediments basinward, throughout the remainder of Cretaceous time, resulted in loading of the underlying sediments, continued growth faulting, and subsequent development of smaller scale horst-graben structures as antithetic faults. Conclusions The following conclusions are made from this study: 1. Based on seismic and geologic data, Late Cretaceous faulting along the east flank of the central Front Range illustrates a new Rocky Mountain tectonic style. 2. The new tectonic style involves the interrelation of shallow depth, penecontemporaneous growth faulting with basement - controlled fault systems. 3. Within the Denver basin, a late Cretaceous depocenter was formed by fault con trol along the east flank of the central Front Range. Rapid deposition of and recurrent movement on the basement- 296

18 Contour Interval = 25'» 19. Isopach map of interval between Transition Zone marker 1 (TZ1) and marker 2 (TZ2) of Pierre Shale (from seismic data). ^:i-^'>f^;^*y.:^^--'- r-^"-y- L :'^ ' ^^.^"^^^^^j^j^*^^"^'~^mffe'?/*tv r '5-' ''^* Vi 297

19 R 71 W Contour Interval = 500 /sec Figure 20. Velocity map of interval between Transition Zone markers 1 (TZ1) and 2 298

20 GEOLOGIC MODEL BASINWARD - HIGH FLUID PRES. FORMATION OF A DEPOCENTER SEISMIC MODEL Figure 21. Seismic anomaly pitfalls. DEVELOPMENT OF A COMPLEX FAULT SYSTEM controlled fault systems resulted in the generation of overpressured shale masses and associated shallow growth fault systems within the uppermost Cretaceous. 4. Similar tectonic features occur elsewhere within depocenters of the Cretaceous foreland basin. In areas of extensive younger Laramide tectonic overprinting, the recognition of early growth fault systems will be more difficult. Integrated geologic and seismic investigations are the only way by which growth fault systems can be identified and explored for mineral resources. 5. Recognition of growth fault systems will play an important role in future petroleum exploration and development in the Rocky Mountain region. Growth faults provide early traps for migrating gas and may control the thickness and quality of reservoirs. Because of tensional antithetic faults and fractures associated with the listric normal faults, production may be enhanced by the *arly developed fracturing. Figure 22. Geologic model for growth fault system in Upper Cretaceous of Rocky Mountain region. 6. Coal exploration and development within the lower Laramie has been greatly affected by influence of growth faulting on coal bed thickness. Growth fault concepts may play an important role in developing coal production associated with delta plain environments of deposition in many areas of the Rocky Mountain region. References Bedwell, J. L., 1974, Textural parameters of clastic rocks from borehole measurements and their application in determining depositional environments (Ph.D. thesis): Golden, Colorado School of Mines, 215 p. Bruce, C. H., 1973, Pressured shale and related sediment deformation: mechanism for development of regional contemporaneous faults: Am. Assoc. Petrol. Geologists Bull., v. 57, p

21 Colton, R. B., and Lowrie, R. L., 1973, Map showing mined areas of the Boulder-Weld coal field, Colorado: U.S. Geol. Survey Misc. Field Studies Map MF-513. Curt-is, D. M., 1970, Miocene deltaic sedimentation, Louisiana Gulf Coast: Soc. Econ. Paleontologists and Mineralogists Sp. Pub. 15, p j :' Davis, T. L., 1974, Seismic investigation of late Cretaceous faulting along the east flank of the central Front Range, Colorado (Ph.D. thesis): Golden, Colo. School of Mines, 65 p. Fenneman, N. M., 1905, Geology of the Boulder District, Colorado: U.S. Geol. Survey Bull. 265, 98 p. Haun, J. D., 1968, Structural geology of the Denver basin-regional setting of the Denver earthquakes: Colo. School of Mines Quart., v. 63, no. 1, p Obradovich, J. D., and Cobban, W. A., 1975, A time-scale for the Late Cretaceous of the Western Interior of North America, in Caldwell, W. G. E. (ed.), The Cretaceous System in the Western Interior of North America: Geol. Assoc. of Canada Sp. Paper No. 13, p Rahmanian, V. D., 1975, Deltaic sedimentation and structure of the Fox Hills and Laramie Formations, Upper Cretaceous, southwest of Boulder, Colorado (M.S. thesis): Golden, Colo. School of Mines, 83 p. Scott, G. R., and Cobban, W. A., 1965, Geology and biostratigraphic map of the Pierre Shale between Jarre Creek and Loveland, Colorado: U.S. Geol. Survey Quad. Map GQ-151. Spencer, F. D., 1961, Bedrock geology of the Louisville Quadrangle: U.S. Geol. Survey Quad. Map GQ-151. Trimble, D. E., 1975, Geologic map of the Niwot Quadrangle, Boulder County, Colorado: U.S. Geol. Survey Geol. Quad. Map GQ Tucker, P. M., and Yorst, H. J., 1973, Pitfalls in seismic interpretation: Soc. Exploration Geophysicists, Monograph 2, 50 p. Weimer, R. J., 1973, A guide to uppermost Cretaceous stratigraphy, central Front Range, Colorado: deltaic sedimentation, growth faulting and early Laramide crustal movement: Mtn. Geologist, v. 10, p