Jour. Research U.S. G(I()I. Survey VoL 5. No. 3. May-June 1911. p. 366-312 DEPOSITIONAL ENVIRONMENTS AS A GUIDE TO URANIUM MINERALIZATION IN THE CHINLE FORMATION, SAN RAFAEL SWELL, UTAH By ROBERT LUPE, Denver, Colo. Abstract.-Uranium deposits in the San Rafa(>l Swell are re­ the Uinta Basin, the outcrops in the Swell are the lated. to sedimentary depositional en.ironments in the Upper northernmost exposures of the Chinle on the Colorado Triassic Chinle Formation. The sedimentary textures result­ Plateau. ing from depositional processes operating in low-energy en­ vironments appear to haV'e influenced uranium mineralization. The Chinle consists of three fining-upward, flu.ial-lacustrine OBJECTIVE sequences. Uranium minerals are concentrated in the lower part of the lowest sequence in areas where sediments of low­ The objective of this study was to determine the energy environments are complexly interbedded with sediments areal and stratigraphic distribution of depositional of other environments. Areas favorable for uranium explora­ environments in the Chinle Formation and their rela­ tion exist in the subsurface to the north, west, and south of the tion to uranium host rocks. Depositional environments Chinle outcrop in the Swell. This determination is based on the spatial distribution of depositional environments and the were interpreted, and rock textures and compositions pattern of Chinle depoSition through time. were noted at each station shown in figure 1. Transport directions were also deterIuined and were used to project outcrop observations into areas where the Uranium deposits in the San Rafael Swell, Utah, are Chinle was not exposed. spatially related to rocks representing certain deposi­ tional environments in the Triassic Chinle Formation. RESULTS This relationship can serve as an important guide in selecting areas favorable for uranium exploration. The stratigraphic distribution of rocks of various ~fany researchers in the Colorado Plateau during depositional environments is shown in cross sections the uranium "boom" of the fifties concluded that the A-A' and B~B' (fig. 2). The Chinle Formation con­ texture and composition of the host rock helped con­ sists of three fining-upward sequences, numbered in trol uranium mineralization (Finch, 1959, p. 144; ascending stratigraphic order, 1 through 3, with each Fischer, 1974). The rock compositions and textures re­ sequence generally finer grained than its predecessor. suIted from sedimentary processes unique to each dep­ The sedimentary rocks also grade laterally from a main ositional environment; hence, the distribution of the area of deposition into finer · grained rocks. rocks of certain depositional environments may be Each sequence generally grades upward and laterally similar to the distribution of uranium lwst rocks. The from proximal braided-stream sandstones and con­ distribution of uranium deposits is also sin1ilar to the glomerates, through distal braided-stream sandstones, distribution of both collapse features and potential to flood-plain or overbank fine-grained sandstones, silt­ source rocks. These factors will be discussed briefly. stones, and mudstones, and~ finally, to lacustrine silt­ This report is the first part of a study of the sedi­ stones and mudstones. The sequences are commonly mentary geology of uranium-bearing Triassic rocks of capped by paleosols. This sequence of depositional the Colorado Plateau. environments reflects a gradual change to environments of lower energy as deposition continued. The sequences GEOLOGIC SETIIN'G also indicate either an increase in distance from the The San Rafael Swell is a doubly plunging, asym­ source area or a decrease in relief during later stages of metric anticline formed in early Tertiary time (Gilluly, deposition. 1929, p. 126); the Chinle Formation is exposed The lateral and vertical distributions of depositional around its flanks (fig. 1). The Swell lies south of the environments are similar. The lateral decrease in grain Uinta Basin, and, except for a few outcrops north of size indicates a trend toward lower energy depositional 365 366 DEPOSITIONAL ENVIRONMENTS AND URANIUM l\IIXERALIZATION. UTAH 111" I sllr UIE c~rL-1 ~~~u.;~-''-----::C:-;::O';''';LO~R~ADO;::;'--- • CASTlEDAlE i i i i ; j SAN RAFAEL SWELL .eIEEI ~<I: ! ;!J II,n >' ~lLl.... ""'" .. """ i : I ! I i i 37" ! ··-·-"ARiiONA·-··----4-·-·Fiiiiii'T•• N W MEXICO' ! I • Boundarv of San Rafael Swe" ! c',) 38" I , 30' ~ , I EXPLANATION '\, I! . ~ Anticline - Showing direction of plunge ! A/> / B o 100 200 KILOMETERS o 10 20 KILOMETERS I I I I I I I iii o 100 200 MILES o 10 20 MILES FIGURE 1.-Locations of the San Rafael Swell, cross sections A.-A.' and B-B', and sample stations. environments away fron1 the main locus of deposition poor. Flood-plain sediments show abundant sub­ in the middle part of the Swell (fig. 2). aerial features such as mudcracks, tracks, and The diagnostic criteria used to distinguish various sediment tubes disgorged by feeding animals. depositional environments are as follows: They are often rhythmically bedded, show unidi­ rectional flow features, and are coarser grained 1. Proximal braided-stream deposits (fig. 3A) are and less burrowed than lacustrine beds (fig. 30). conglomeratic sandstone containing ripped-up intraclasts near their strongly erosional bases. 4. On the other hand, lacustrine sediments lack bed­ They reflect the highest transport energies dur­ ding because of heavy burrowing, show little ing Chinle deposition. Their sedimentary struc­ evidence of subaerial exposure, and are generally finer grained than flood-plain deposits (fig. 3D). tures are relatively simple~ consisting mainly of unstructured and flat beds. There is little vertical 5. Paleosols (fig. 3E), common at the top of these change in grain size. Proximal braided-stream fining-upward sequences, are both calcareous and rock bodies are lenticular in cross section, and siliceous. They formed in a semiarid climate foreset bedding and channel axes indicate unidi­ (Cooke and ,,\Varren, 1973, p. 114). rectional transportation. One feature deserving special attention is the c.om­ 2. The distal braided-stream deposits, which usually plex interbedding of rocks deposited in various envi­ overlie and laterally abut the proximal braided­ ronments near the north and south ends of the Swell strean1 deposits (fig. 2), contain sand-sized sedi­ (fig. 2). In these areas, the sediment bodies are also ment that is finer grained than the proximal generally smaller. This complex interbedding may have deposits and that is commonly typified by foreset played an important part in control of uranium min­ bedding of transverse bar origin (fig. 3B) eralization. (Smith~ 1970, p. 3000; Asquith and Cramer, 1975, In reeonstructing the areal distribution of deposi­ p. 660). Sediment transport in these deposits tional environments, it is helpful to examine transport was also unidirectional. directions for specific horizons in the Chinle. Trans­ 3. Flood-plain or overbank deposits can be confused port directions for the lower part of sequence 1 . are with lacustrine sediments where outcrops are shown in figure 4. The rose diagram defines a strong LUPE 367 SOUTH NORTH E ... A :rop of Triassic Chinle Formation A' It) t.. 12 11 23 13 14 15 25 24 16 10 21 17 20 9 / I _. ---=--.: ----------~ SOUTH NORTH E ... B ;rop of Triassic Chinle Formation B' It) t.. 12 11 22 2626A 18 9 I I o 0-.... 8 It),..- "" o 5 10 15 20 KILOMETERS I I I I I I I I I o 5 10 15 20 MILES EXPLANATION SEDIMENTARY DEPOSITS URANIUM DEPOSITS k:.:: :.::~ Proximal braided stream • >100,000 tons of ore produced, through 1957 k\:r:y / I Distal braided stream • >10,000 tons of ore produced, through 1957 8---------3 Floodplain • Prospect l::-;:::~ Lacustrine Paleosol Contact ~ Unconformity FIGURE 2.-Cross sections (A.-A.' and B-B') showing sedimentary depositional environments and the stratigraphic distri­ bution of uranium deposits in the Triassic Chinle Forma tion, San Rafael Swell, Utah. Xumbers indicate sample sta­ tions shown in figure 1. The lower part of sequence 1 is roughly equivalent to the Moss Back )Iember of Stewart, Poole, and Wilson (1972, p. 31). Uranium data are from Hawley, Robeck, and Dyer (1968). northwesterly transport trend 'with minor divergence. A paleogeographic reconstruction of the lower part Geographically, that trend is represented by a corridor of sequence 1 is shown in figure 5. These environ­ of deposition across the central part of the Swell ments, generalized from figure 2, were tied together (fig. 4). The divergences from the trend in this in- schematically by using transport trends. terval of the Chinle are in the north and south ends These studies indicate that a system of braided­ of the Swell where streams trended slightly away stream sediments trended toward the northwest across the central part of the Swell during early Chinle from the axis of the corridor. The pattern suggests time. At the north west end and off the flanks of the that currents were weaker in the north and south. corridor, currents waned and lower energy environ­ This inference is supported by the greater abundance ments prevailed. Furthermore, these flanking distal of low-energy environments in these areas (fig. 2). areas were more complexly interbedded. 368 DEPOSITIONAL ENVIROX~lENTS AND URANIUl\1 l\1INERALIZ.ATIO~, UTAH LUPE 369 111'00' 45' 110'30' N EXPLANATION I --..... Transport trend I 39' 00' J w ~ __-----E 38' 45' 10 20 KILOMETERS I I i I A . S B 10 20 MILES FIGURE 4.-Transport directions of the lower part of sequence 1 of the Chinle Formation, San Rafael Swell. Utah. A, Equal­ area rose diagram of the total of 130 measurements. B, Geographic distribution of transport trends. The patterned area suggests the stream dispersal pattern during deposition of the lower part of sequence 1. URANIUM ship between depositional environments and uranium. The distribution of uranium deposits in the San The areas to the north and the south are extensions Rafael Swell is shown in figures 2 and 5.