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An Alluvial Fan Model for Mineral Exploration, Report of Investigations No Waste Isolation Pilot Plant Compliance Certification Application Reference 428 McGowen, J.H., and Groat, C. G. 1971. Van Horn Sandstone, West Texas: An Alluvial Fan Model for Mineral Exploration, Report of Investigations No. 72. Bureau of Economic Geology, Austin, TX. Submitted in accordance with 40 CFR 5194.13, Submission of Reference Materials. - t*- An Allurjial Fan kdel for Mineral Exploration J. H. McGowen and C. G. Groat BUREAU OF ECONOMIC GEOLOGY The University of Texas at Austin Austin, Texas 78712 W. L. Fisher. Director Report of Investigations-No. 72 Van Horn Sandstone, West Texas: An Alluvial Fan Model for Mineral Exploration BY J. H. McGowen and C. G. Groat 1971 Second Printing, December 1975 Third Printing, October 1982 Abstract . .......... Proximal fan ........ Introduction ........... Mid-fan .......... General description .......... Distal fan ......... Geologic setting ........... Main braided channels .... Van Horn Sandstone: a large alluvial fan system Braided distributaries ..... Sedimrntology ........... Braided interlobe area .... Facies ............. Summary ........... Proximal f'acies......... Compositionandmineralogy ...... Massive l~oulder1. cds ....... Gravel composition ........ iilterriating gravel and thin sand beds . Sandstone petrography ....... Thick sand and muddy sand of the proxi- Quartz ........... mal facics .......... Feldspar ........... Other feat.urcs of the proximal facies . Rock fragments ........ Mid-fan l'acies .......... Hematite .......... F:tcies dcscrlptioli ........ Cement and matrix ....... Distal fitcies ........... Provenance .......... Main hraided channel subfacies ... Clay mineralogy ......... Br~titled(itstributary subfacies .... Heavy minerals ......... Int.erlotw !)raided subfacies ..... Economic geology ......... Deposittonal trends ......... Gold ............. Depositional processes arrd environments . Prospecting model ......... Sedimentary processes and facies .... References cited .......... Appendix: Bed forms and stratification types ILLUSTRATIONS F 1c;rr rt~s PAGE FIG~JRES.. I . Map shnn.ing Ioc.. tiotj 9CVari Horn Sandstone 14. Channel profiles generalized from gravel- alid outc~.up,its source t-.ri.ane, and related struc- sarid-filled channels of the mid-fan facies turd I'c. .it arc.. .... ... 3 15. Thick foreset crossbeds of granular very coarse 2 Otttcrop :\lit1 ioc:tlity!.cc tton map of t.he Van sand (Neal ranch. section P4) ...... t3or11 S;~~i:ls;i~iicl !I~tc!spcth aird ('u!herson 16. Characteristics of the mid-fan facies (McVay I is. 'I ....4 ranch. section MV17) ......... 3. Schrr11;ltlt- i>:;ii- oi' t';iti I!ori~ facies ... S 17. Gravel-filled channel of the mid-fan facies 1. Perccnl g-ravci .I, ou;rropping Val1 Horn Sand- (McVay ranch. section MV17) ...... SlotLC . ....... 11 18 . Sand-filled channel of the mid-fan facies 5 . Cross serl iori I '16 i>!tgki ticrrthernmost Val1 Horn (McVay ranch. section MV17) ...... S;~ndstonc011 1!1c ficene ranch (localities K1- 19. Compound foreset crossbeds (McVay ranch. K1) ............... 12 section MV14) ........... 6 . L)eposiiional t;.c,titis<>!'!iic Van Horn Sandstone . 13 20 . Stratification in sands of the mid-fan k~cies 7. Characle~tztic.:. i:1' !I:(. ;)roximal facies on the (McVay ranch. section MV15: ...... Sea! r:rncit ...c.c~ionP! .......... 14 31. Percent of Van Horn Sandstone comprised of 8 . (:l~:rract<>r~\~ic>(. ! rhv proxima! facies on the foreset and trough-fill cross strata ..... Ke~i~er:~t~rii (S~~:IOI.I 1. i ! j ..... 15 22 . Upper part of Van Horn Sandstone on the 9 . Ch;ir..rt~ristic:. of the proximal facies on the Keel?(>ranch ............ I<eclic. I .:ill c.h isc.ciio!l !<? I ..... i6 23 . Stratification types representative of distrjl 10 C'har.icteri:. tic\ (.I' the I. zlsal proximal Eacies on facies deposits in the northernmost Van 1Ior.11 the Iiectre rai~cii(scction Ti1 1 ...... 17 <)uLcrops ............ I I . Char. !ctcristics of son:( of the upper parts of 24 . Details of some mid-fan deposits that are inler- thr i. r~.simal f;rcies on tile Neal ranch (section bedded with distal-fan deposits ..... P.t.I ... ............ 18 25 . Distal fan facies of the northern outcrop 12. 1, i)ngitutl.t~.ii qraiiti i,. !rs and flanking sand- :IL locality K3 on the Keene ranch ..... fillcri channr15 ,Ic.i,r tlic southern limits of the 26 . Some characteristic features of the upper Val1 pi.o\i~~~;~li'.icics (11cVa5 ianch. sectioii MV17) . 19 IIorn Sandstone .......... 13. G~.a~t'l.Fiiii:il cil~unne! lickti the upper part of 37 Represrritative sedimentary structures of the Lhc {~oxit?i.I! !:I:.~IJs !Nt>.ii liil?ch,section P4) . 19 braided distributary subfacies ..... FIGURES- PAGE FIGURES- PAGE 28 . Convolulions and injection features of the 32. Composition of gravel in the Van Horn Sand- braided distributary subfacies (McVay ranch. stone ............... 43 sectionMV4) ............ 34 33. Composition of the Van Horn Sandstone ..44 29. Percent of Van Horn Sandstone comprised of 34 . Plot of mean size vs . weight percent of heavy trough-fill cross strata .......... 35 minerals .............. 48 30. Interpreted Van Horn paleogeography .... 38 35. Alluvial fan prospecting model ....... 51 31. Plan and cross section of a single canyon and 36. Characteristic bed forms and stratification fan ................ 39 types in the Van Ilorn Sandstone ...... 56 VAN HORN SANDSTONE, WEST TEXAS: AN ALLUVIAL FAN MODEL FOR MINERAL EXPLORATION J. H. McGowen and C. G. Groat ABSTRACT The Van Horn Sandstone is an ancient alluvial- stream, braided distributary, and braided inter- fan system with a northern highland source area lobe) have been differentiated. Main braided of rhyolite, granite, metamorphic and sedimentary stream deposits occur as foreset and trough cross- rocks. Sediment ranging in size from silt to boul- beds; relative abundance of each crossbed type is ders was transported southward through canyons variable. Scale of foresets and trough crossbeds by high-gradient mountain streams. South of the decreases upward in the section and to the south. mouths of the canyons, sediment was transported Braided distributary deposits are characterized by by less confined braided streams. a relatively high content of muddy sand units, The fan was deposited on a surface of faulted, well-preserved channel-fill deposits, some ripple folded, and highly dissected Precambrian sedi- cross-laminated sand units, foreset- and trough- mentary, metamorphic, and intrusive basic igneous crossbedded sands (locally the facies may be domi- rocks. Topography of the subjacent erosional sur- nated by small trough crossbeds), and units dis- face was rugged and characterized by deep canyons playing soft sediment deformation and injection that controlled the distribution of gravel downfan structures. Braided interlobe deposits consist of beyond the termination of high-gradient mountain thick sand sequences composed almost exclusively feeder systems. of trough crossbeds. Muddy sand is uncommon Three principal facies of the Van Horn area are, and trough laminae are accentuated by heavy from north to south, (1)proximal, chiefly gravel; mineral placers. This subfacies contains the great- (2) mid-fan, alternating gravel and sand; and est heavy mineral concentration within the Van (3) distal, dominantly coarse and very coarse sand. Horn Sandstone. The proximal facies is 300 to 600 feet thick in the Facies relationships, textural trends, succession northern outcrop area and consists of lower thick, of sedimentary structures, and dip direction of massive cobble and boulder beds and upper thin to crossbeds indicate a southerly sediment transport thick, horizontally bedded pebble, cobble, and direction. Depth of flow, slope of the fan surface, boulder gravels. It grades southward and vertically stream velocity, and competence decreased south- into alternating gravel and sand of the mid-fan. ward. Proximal facies developed under confined, Mid-fan deposits are up to 300 feet thick in the rapid flow condilions; surges were common events northern outcrop area and about 30 feet thick in within the feeder system and within the canyons. the central area where the facies plunges south- Mid-fan deposits were laid down by braided ward into the subsurface. Mid-fan gravels occur as streams; both rapid and tranquil flow conditions lenses of parallel-bedded depositional units with are indicated by longitudinal gravel bars and by flat bases and convex upper surfaces and as channel- flanking transverse bars with dunes, respectively. fill deposits. Sands associated with the mid-fan The distal fan was the area of lowest physical occur either as broad channel-fill deposits or as energy. Tranquil flow conditions dominated, lenses deposited in low areas flanking gravel lenses slope of the surface was extremely low, and sedi- (or bars). Two stratification types, foreset and ment remained saturated for relatively lengthy trough crossbeds, dominate these sands. periods following a depositional event. The distal facies consists chiefly of coarse and The Bliss Sandstone (Ordovician) is the oldest very coarse sand; the mid-fan facies grades south- sedimentary rock sequence that overlies the Van ward downfan into distal facies, which also over- Horn. In places the contact between Bliss and Van lies mid-fan deposits throughout the fan. Maxi- Horn is discordant, but in others the contact is mum outcrop thickness of distal facies is about conformable. 700 feet. Three distal fan subfacies (main braided Although placer gold had been reported in the 2 Report ofInvestigations-No. 72
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