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GEOLOGY OF THE TAYLORSVILLE AREA, NORTHERN SIERRA NEVADA

By Vernon E. McMath Continental Oil Co., Ponca City, Oklahoma

In the northeastern Sierra Nevada, the great batho- Thickness determinations are only approximate since lithic edifice of the High Sierra ignominioush* loses the strata have been thinned plasticall\', thickened by its footing amidst terrain dominated by volcanic geo- minor folds, faulted b\ numerous indeterminate Ccno- s\nciinai fill. The eastern marginal fault s\steni of the zoic normal faults, and obscured by extensive forestT^ High Sierra gives way to a series of lesser Basin and Range structures. The degree of metamorphism be- 120* comes less. But it is these very diminutions, especiallv

, Lake Almanor the paucity of plutons, that account for the signifi- cance of this relativeh- small area in the northernmost Area of block diagram Sierra Nevada; the geos\nclinal record has not been 7 Taylorsville Area obliterated. The significance of this record was dem-

onstrated by J. S. Diller in a series of papers culminat- ing in U.S. Geological Survev Bull. 353, Geology of the Taylorsville Region ( 1908).^his region, discussed

herein, is bounded on the south by the 40th parallel Permian? 8 Mesozoic (fig. 1), but much of the Paleozoic section continues Wedge beyond the parallel, southward into the adjacent Downieville 30-minute quadrangle (Turner, 1897)7\ Regionally but lightly metamorphosed Paleozoic, Pyroclastic Sequence Triassic, and Jurassic marine rocks, all deformed in the Nevadan orogeny, underlie most of the Taylors- ville area. The younger granitic plutons and still Shoo Fly Formation younger Tertiary volcanic rocks, though locally prom- inent, are excluded from further consideration here. Because the older stratified rocks are metamorphosed, the prefix "meta" should be attached to the names of all types except those intrinsically connotating meta- morphism. For the sake of simplicity, however, the

prefix is omitted but is understood to apply to all the pre-Nevadan rocks discussed here.

[Regional metamorphism is of the greenschist facies] although locally a prehnite-albite assemblage suggests the lower grade prehnite-pumpellyite metagray\\acke facies (Coombs, 1960, 1961). JVIetamorphic foliation ranges from well developed in phyllites to impercep-

tible in lava floA\s, sills, and some tuffs and pyroclastic

breccias. It is generally an axial plane cleavage and is more consistent in attitude than bedding. Locally, bed- ding has been overturned beyond the position of the axial plane so that bedding-cleavage relations indicate an incorrect direction of top. 0\\ing to penetrative deformation, the degree of elongation of originall\- 20 MILES equidiniensional fabric elements may be as much as Figure 1. Index map showing the relations of the Taylorsville area three- or four-fold but is generally less. much Where to the regional geology. Taylorsville (T), Quincy (Q), Wodes Lake (W), original elements have been elongated, the\' lie in the Sierra Butles (S), Downieville (D), North Fork American River where it crosses the top of the Shoo Fly Formation (N), Colfox (C), and plane of metamorphic foliation and plunge steeplv. Placerville (P) are represented on the map by letters.

[173] 174 Gf.ol(x;y of Nortiiirn California Bull. 190

Photo 1. View southeast acros! Indian Valley. Mt. Hough, right skyline, shows exposures of Peole and Arlington Formations of the lower plate of the Toylorsville thrust, rizzly Peak, left center skyline ridge, shows outcrops of upper plate Shoo Fly (Toylorsville Member), Grizzly, and Sierra Buttes Formations. Mt. jra, left, exposes Jurassic rocks of the lower plate. Indian Valley is a complex graben. Camera site is under- lain by Shoo Fly of the upper plate.

At the present state of understanding of the evolu- syncline was continuous involves interpolation of 17.^ tion of the Sierra Nevada, it appears that the Taylors- miles, but the large outcrop breadth of 20 to ."JO miles ville area lies athwart the Ncvadan structural axis. combined with the continuity of the west limb in the Farther south in the central Sierra Nevada, this axis interval of interpolation permit little room for alter- is the locus of the batholithic complex. In the north, natives. A synclinal axis inferred by Taliaferro (1942, the axis is that of a major overturned syncline in which p. 99-100) in .\Iesozoic rocks west of Lake Tahoe the entire know n stratigraphic section is involved. The de.scribes an intermediate point. Silurian, Mississippian, Permian, Triassic, and Jurassic This Nevadan s\ncline within the Taylorsville area and possibly the Devonian systems are represented. is overturned to the northeast, and the overturned That the axis of batholithic activity coincides with a southwest limb has been thrust northeastward along major syncline has been shown by Rinehart and others the Taylorsville thrust onto the synclinal axis (see

(1959, p. 941), and they have added the Ordovician fig. 2). The upper plate can be traced onl\- 5-10 miles and Pcnns>lvanian to the list of systems represented. south of the 40th parallel before it is obliterated by All the pre-Nevadan stratified rocks of the Sierra Ne- granite or covered by Tertiar\' volcanic rocks. To the vada may be interpreted as defining a faulted s_\n- east, the moderately dipping normal limb contains only clinorium (Bateman and others, 1963, p. D-6), for Mesozoic rocks as granite occupies the position where which the Nevadan sxnclinc is the axis. That the one would expect to find the Paleozoic section. To the 1966 McMath; Taylorsvillf. Area. Northern Sierra Nevada 175

Figure 2. Generalized btocic diagram of the Toylorsville area. vrn» n«TE Teeth denoting the upper plate of the Toylorsville thrust hove been employed even where the thrust has been offset by younger faults. The Kettle Formation, which mokes up much of the east limb of the Smirlnitr Forntlon •od Nevadon syncline, is designated by two approximately equivalent Na»i«Tliui LlHitont composite strotigraphic units: (1) Lilac through Moonshine Formations, UNCONFORUITY and (2) unnomed dacite tuff-breccia through Cooks Canyon Formotion.

Jura is Mt. indicated by a hochured circle in the half fenster de- 'V'*!"'— scribed by the Toylorsville thrust, west, the Paleozoic of the overturned limb was faulted relatively upward along a steep strike fault, so that it is not now possible to trace the upper plate westward toedhut triinttoiit to its root zone. To the north, both limbs and the upper plate of the thrust are concealed by the Tertiary lavas of the Modoc Plateau.

MAIN STRATIGRAPHIC SEQUENCES Ntli Feiut Ion Three and probabl\' four demonstrable stratigraphic ?7^ sequences make up the west limb of the Nevadan Tiyler Forntlon syncline (see fig. 3). "Sequence" is used in the re- stricted sense of a stratigraphic unit bounded above and below by major unconformities as proposed by Sloss and others (1949, p. 110) and as applied in the Cordilleran geosyncline by Silberling and Roberts (1962). Application of the "sequence" concept in the iltilir Forut Ion Taylorsville area has the further advantage that the UNCONFORUITY unconformity-bounded units are lithogenetically dis- tinct from those above and below and also show a tiloo Fly Fofut lo high degree of internal lithologic uniformity. 17rt G|-0L(K;V ok NoRTHlRN Cm. NORMA Bull. 190

ModitKd from Turnir (IS97) Thit Paptr Modidad Irom Dilllr (1906) Oo«n(tvllli OuodrongK Plott Upp«r Plott SWEARINGER SWF.ARINOER

HOSSELKUS HOSSELKUS

MILTON

(in port)

Carboftiftrou

QUARTZ PORPHYRY

Cortoniftroul \ CALAVERAS MONTGOMERY

GRIZZLY

METARHYOLITE Prt-SilunOH Sitrro Butltl

Figure 3. Pre-Jurassic correlation diagram for the Toylorsville area. Diller's column (on right) is modified In that his easti rn belt of Taylor

Meto-Andesite is inserted between his Peole and Reeve as demanded by his map relations, and his Robinson and Reeve Forma ions are indicated as Permian as emended by Thompson ond others (1946). Turner considered his augite porphyrite and quartz porphyry to be Juratrias in age. but they are shown as Permian as emended by Wheeler (1939).

The lowest sequence consists of only two named sippian and Permian horizons. This sequence provides formations, the Shoo Fly an(d the Taylorsville, which the ke\' to both the structure and the stratigraphy; as mappecd by Diller hroadl\- represent respectivel)' hence, it has been the most thoroughh' studied and lower and upper plate equivalents. Neither the lower w ill be the principal subject of this paper. limits nor the stratigraphic makeup of either unit is The third sequence, found only in the upper plate, well known except in local areas. Because the Shoo contains strata of Triassic age only, hence may ade- Fly constitutes a large part of the outcrop belts for- quately be referred to as the Triassic sequence. It merly referred collectively to the Calaveras Group consists of two fossiliferous shelf-type formations, the (Clark and others, 1962, p. B-17), has a much greater Hosselkus Limestone and the Swearinger Formation. extent along strike (100 miles or more), and shows The Hosselkus rests with angular unconfonnity on promise of a more undisturbed and complete section the p\roclastic sequence. The Triassic sequence is (possibly 40,000 or 50,000 feet), the name Shoo Fly rclativel)' thin, having a preserved thickness of only is provisionally extended to include the upper plate 1,100 feet. However, its upper boundary is the trace equivalents. Slate or phyllite, chert, and sandstone, the of the Taylorsville thrust and it is locally cut out by dominant components, contrast markedly with the a granitic pluton, hence it may well have been unconformahl\- overlying pyroclastic sequence. Silu- thicker. That the sequence probably is separated from rian fossils are present near the top. the overh'ing sequence of Jurassic rocks by an un-

The second sequence, here termed the pyroclastic conformity is inferred from relationships demon- sequence, is found in both the lower and upper plates. strated farther south in the Sierra Nevada, where Clark

It comprises eight formations consisting dominanth- and others (1962, p. B-19) have shown that the Lower (95-98 percent) of coarse water-laid, presumably Jurassic apparently oversteps about 400 feet of Upper

Triassic(?) limestone within 1 mile. marine, pyroclastic debris. It is nearly 25,000 feet The fourth and youngest sequence consists only of thick and rests with angular unconformity on the Jurassic strata, hence again is adequateh' referred to Shoo Fly Formation. Two fossil localities, one near as the Jurassic sequence. It is about 1 5,000 feet thick the middle and the other near the top, provide ages on the east limb and probabh- a little thinner on the of Mississippian and Permian respectively. \\'cst limb of the Nevadan syncline, and is known only Informal reference to the pyroclastic sequence of in the lower plate. The widely known sections on Plumas County may prove useful; formal designation Mount Jura have been referred to informall\- as the as the Plumas sequence would be desirable were it Mount Jura sequence for many \cars, but formal

not for the possibility that a significant but undetected designation is considered neither nccessar\- nor de- unconformity may intervene between dated Missis- sirable at this time. —

1966 McMath: Taylorsville Area, Northern Sierra Nevada 177

The Jurassic sequence is dominated by volcanogenic The lower part of Diller's Paleozoic sequence is roclcs, although epiclastic or reworked volcanic rubble based on apparent superposition above and below a and possibly nonvolcanic epiclastic material is inter- single dated (fossiliferous) horizon, his Silurian Mont- clalated here and there in the section. Indeed, orogcnic gomery Limestone. His four older formations are now

(polymictic) conglomerate is prominent in the upper recognized to lie in the upper plate, and his three part of the section, which may well be equivalent to N'ounger formations constitute the lower plate. The the .Mariposa Formation of the foothill belt. Though uppermost formation of this sequence of seven as es- the thickness of volcanic material suggests a deep tablished b\' Diller was the Shoo Fly Formation. Be- trough, the general aspect is one of shoaling compared tween the Shoo Fly and the upper part of his Paleozoic to that of the pyroclastic sequence. Perhaps this ex- sequence, Diller indicates a break. Above this break, plains why the Mount Jura sequence is unusualh' fos- he placed the three fossiliferous (late Paleozoic and siliferous for a eugeosynclinal sequence. Triassic) formations of the upper plate in their cot^

Study of this sequence is incomplete; hence this rect relative order. Owing to paleontological control, treatment is little more than that of cataloging some he correctly recognized that these three fomiations of the more significant conclusions. were overturned, but he could not recognize that the seven below the break were also overturned. REVISION OF PALEOZOIC STRATIGRAPHY Durrell (/;/ Durrell and Proctor 1948, p. 171) de-

The first geologic column of the area was provi- scribed the first irrefutable contact relationship be- tween two of the formations of Diller's Paleozoic sionally set up by J. S. Diller (1892, p. .^72) in a pre- liminary account of the earliest s\'stematic study of succession. Near Wades Lake in the center of the the Taylorsville area. At the conclusion of intermit- Downieville 30-niinute quadrangle, Diller's Shoo Fly tent field work, a modified and e.xpanded column was Formation (equivalent to and having priority over presented in a more complete paper (Diller, 1908, pi. Turner's Calaveras Formation) is clearly overlain with

4) and this is reproduced here as a part of figure 2. angular unconformity by the Sierra Buttes Formation Major discrepancies between Diller's sequence and (new name for the metarhyolite series of Durrell, that of Turner (1897) in the adjacent Downieville quartz porph\ry of Turner, and metarhyolite of .^0-niinute quadrangle are readiK' apparent despite the Diller). Detritus from the Shoo Fl\- is included in the fact that Diller and Turner worked contemporane- basal conglomerate of the Sierra Buttes Formation and ously on a project that involved both areas. For ex- varies from place to place according to the underlying ample. Turner's east-facing Juratrias sequence of Shoo Fly rock type. Well exposed in the same area quartz porphyry and augite porphyrite was described is a gradational contact between the Sierra Buttes and by Diller as west- facing Silurian (or pre-Silurian the overlying Taylor Formation, w hich Turner ( 1897) text and plates are contradictory) metarhyolite and called augite porph\'rite. Graded bedding in both the Carboniferous Taylor Meta-andesite, respectivelv. Sierra Buttes and the Taylor confirm the succession. It is apparent that a major difficulty facing the early Reconnaissance by Cordell Durrell into the Taylors- geologists was the lack of paleontologic dating. Only ville area showed the same succession, but overturned: in light of later advances made in stratigraphy could Shoo Fly, Sierra Buttes, and Taylor. The Sierra Buttes the geology be confidently pieced together without there is thin and was apparently overlooked by Diller. the aid of fossils. These advances consist of the recog- The Arlington Formation of Diller is adjacent to the nition of facies change and of the significance with Taylor and seemed to constitute a fourth and even respect to "top and bottom" of many small-scale younger fomiation of this sequence, for graded bed-

(geopetal) structures, of which graded bedding is the ding indicated that it too is overturned. most widespread and useful in this area. Drag folds Subsequent field studies b\' the writer showed that and the angular relationship of cleavage to bedding Diller also failed to identif\- his Peak Formation be- have been of only secondary usefulness but in some tween the Arlington and Taylor. The contact between places are misleading. the Sierra Buttes and the Shoo Fly is clearly an angular The discrepancies between the Downieville quad- unconformity in \\'hich a section of the Shoo Fly sev- rangle and the Taylorsville area have now been largely eral thousand feet thick is truncated. No fossils that reconciled as one result of drastic revision of Diller's would permit an age assignment have yet been found. structural and stratigraphic interpretation (fig. 2). This is the lower plate succession and is tabulated in Turner's east-dipping Juratrias volcanics have proved figure 2 for comparison with Diller's succession. to be components of the late Paleozoic pyroclastic Another Paleozoic succession, which differs from sequence and are overturned in the north part of the the lower plate succession owing to facies change, was Downieville quadrangle. They continue northward faulted to the northeast along the Taylorsville thrust. into the Ta\'lorsville area retaining their overturned Here again, graded bedding indicates that the succes- position and are there repeated by a major low-angle sion is generally overturned and that it faces to the fault, the Taylorsville thrust. The repeated section is northeast as does the lower plate succession. now also recognized in the northernmost part of the In the upper plate, the Taylorsville Formation of Downieville quadrangle. Diller occupies a stratigraphic position analogous to 178 CiKOUXiY OK NoRTllKKN CaI.IIORMA Bull. 190 that held in the lower plate !)>• the Shoo Fl\' Forma- now known to be Mesozoic in age, and rocks that tion. The type Taxlorsville is here provisionally low- might be candidates for designation as t\pe Calaveras ered to member status within the Shoo Fl\-, and the arc structurally very complex (Clark, 1964, p. 8, 12). name "Shoo Fl\" applied to all upper plate rocks .As at least a temporar\- expedient, the term "Cala- lying unconformably below the pyroclastic sequence. veras" will not be applied to an\- of the Taslorsville The fossilifcrous Silurian Montgomery limestone beds rocks. are intercalated near the top of the Ta\lorsville Mem- DESCRIPTION OF THE SEQUENCES ber. Diller (1908, p. 18-19) inferred an unconformity Shoo Fly Formation between the Montgomery and Taxlorsville on rather insecure evidence, and not knowing that the section The Shoo F"l\' Formation underlies ncarl\' the entire area of the largest most fault-bounded is overturned, he a.ssigned a probably Devonian age and continuous to the Ta>lorsville. Paleozoic belt on the geologic map of California (in pocket), at least as far south as Placerville (see fig. I). The ne.xt overlying formation in the upper plate is the Grizzl)-. .^n angular unconformity, the same as .At the present stage of investigation, the Shoo Fly is that between the Shoo Fl\- and Sierra Buttes Forma- formally a formation, but it certainK will eventuall)- tions, separates it from the underlying Ta\lorsvillc. be given formal sequence or at least group status.

The epiclastic Grizzly is interpreted as a basal unit Three lithic units have already been discerned in the deposited locall\- on an irregular erosion surface before upper 10,000 feet of the lower plate. the beginning of pyroclastic volcanism. It is overlain The uppermost of the three is greenstone, w hich is in turn by the pyroclastic Sierra Buttes Formation .^,000 feet thick and localh' truncated by the overlying (Diller's metarhyolite). p\ roclastic sequence. Clasts of the greenstone are in- Xe.xt in succession, as in the lower plate, are the corporated in the basal few feet of the p\roclastic Taylor and Peale Formations. Diller called the Taylor sequence. of this block the Hull Meta-andesite and believed it The middle unit, about 2,000 feet thick, is almost to consist of Late Jurassic intrusive and volcanic rocks. certainly the lithic equivalent of the Taylorsville Mem- It is. however, clearly pyroclastic and very much like ber, which is in the upper plate and also about 2,000 the type Taylor except that it contains a wider variety feet thick, although its base is faulted out (fig. 2). of sills and dikes and additional varieties of meta-ande- This unit in both plates underlies the pyroclastic se- site breccia. quence with angular discordance, and it consists prin-

Conformably above the Peale lies the Goodhue cipall\- of laminated slate and ph\llite, thin beds of Greenstone (new name) which Diller erroneoush- fcldspathic, p()ssibl\- tutTaceous graywacke, and minor mapped as Taylor. The t\pe Taylor lies belo'v: the intraformational breccia, chert, and limestone.

Peale. Aloreover, the rock of the Goodhue is readily The lower of the three units is similar to the middle distinguished from that of the type Taylor. Although hut also contains significant interbeddcd sandstone, the Goodhue ma\' be s\nchronous with the lower which ranges from feldspathic gra\wacke to essen- beds of the lower plate Arlington Formation, the rocks tiall>- orthoquartzite, and some felsitic tuff or chert. are lithologicall\- dissimilar. It is .\000 feet thick. The next two formations are in the same order that Below these three units, very thick undifferentiated Diller placed them: the Reeve Formation, then the slate and phyllite, chert, felsitic tuff, gra\wacke, and Robinson Formation, both of which, however, are greenstone in variable but decreasing proportions in herein redefined. Some Reeve rock types are very about the order listed comprise the bulk of the Shoo distinctive, and their presence among the southeastern Fly of the lower plate. .A few spot observations sug- outcrops of the .Arlington Formation strengthens cor- gest top is to the northeast for the entire lower plate relation of the upper and lower plates. The Robinson .sequence. If the sequence is homoclinal and unfaulted, rock type is broadl\- comparable with that of the bulk it is 40,000 to .>0,000 feet thick. of the .Arlington Formation of the lower plate. The Shoo Fly of the upper plate includes, in addi- Diller (1908) included the Peale, Shoo Fly, Taylor, tion to the Ta\lorsville Member and most of the lithic and .Arlington Formations in the Calaveras Group of t\pes found in the lower plate, a wide arra\- of dikes, supposed Carboniferous age. Turner (1897) included sills, and small plutons that have hindered further the direct extension of Diller's Reeve and Robinson stratigraphic subdivision. in the Calaveras whereas Diller excluded them. Turner To the southw est, the Shoo Fly is generally bounded excluded the Ta\lor, and mapped much of the .Arling- by a major serpentine-belt fault (Clark, 1960). How- ton or .Arlington equivalents in his jMilton Formation. ever, at the northwest end of the belt, just south of

The Shoo Fly together with its synonym, the Blue Lake Almanor, a thick east-facing Mesozoic and Per-

Can\'on Formation of the Calaveras Group (Lindgren, mian(r ) wedge-shaped unit lies between the sepentine- 1900), have recently been excluded (Clark and others, belt fault and the Shoo Fly, and it must be separated

1962, p. B-17), in part because rocks of Silurian age from the Shoo Fl\- b\' another fault, probabl\- a thrust were specifically excluded from the Calaveras by (see fig. 1). A fusulinid locality reported by Diller

Turner (1894, p. 446). Much of the type Calaveras is (1908, p. 23) to lie at the west margin of the Shoo 1966 McMath: Tayi.orsvii.i.i Aria, Northkrn Surra Niaada 179

Fly is probably at the base of the Permian(?) and 1897, p. 2), Diller's metarhyolite (1908, p. 81) and Mesozoic wedge. quartz porphyry (1895), and the metarhyolite series No fossils unequivocally collected from the Shoo of Durrell and' Proctor (1948, p. 171). The Sierra Fl>- of the lower plate are diagnostic of its age (see Buttes (fig. 1), the highest and most distinctive peaks Clark and others, 1962, p. B-17). However, the Mid- composed of the rocks of this formation, are in the dle(?) Silurian Montgomery Limestone of Diller is south part of the Downieville 30-minute quadrangle. intercalated in the Taylorsville Member of the upper .\ t\pe section was not designated, but a representa- plate and this in turn is almost certainly equivalent to tive section can be seen in the vicinity of Long Lake a unit in the upper part of the Shoo Fly of the lower and Wades Lakes seven miles northwest of the Sierra plate. Tlius the Shoo Fly is at least in part Silurian. Buttes. The thickness of the formation is 4,000 to Owing to its very great apparent thickness, it probably 5,000 feet but decreases to less than 1,000 feet in the includes rocks as old as Ordovician. northwest part of the lower plate. Excluding dikes and sills, the formation consists principally of bedded Pyroclostic Sequence quartz keratoph\re breccia, tuff, and perhaps some The p\rocIastic sequence includes in the lo\\er plate flows, whose gross chemical composition is probably the Sierra Buttes, Taylor, Peale, and Arlington Forma- closer to dacite than to rhyolite. .Minor chert, slate, tions, and in the upper plate the Grizzly, Sierra Buttes, and rare limestone with fragments of marine fossils Ta_\'lor, Peale, Goodhue, Reeve, and Robinson Forma- also are present. tions. Taylor Forviatioii. The Ta\lor Meta-andesite of The p\'roclastic nature imparts a lithogenetic unity Diller (1908, p. 83), or the augite porphyrite of to the sequence and presents a strong contrast with Turner (1897), is characterized by augite andesite the sequences above and belo\\-. A marked and signif- breccia, tuff-breccia, tuff, subordinate flows, minor icant feature of the pyroclastic sequence is the relative black tuffaceous slate, and crinoidal limestone. Its sharpness and contrast owing to the change of chem- thickness is about 8,000 feet. The size of the blocks in ical composition from one formation to the next. The the breccia increases to 6 to 8 feet in the Downieville gross succession of approximate rock types is dacite, 30-minute quadrangle, hence the source probably lay andesite, latite, basalt or andesite, and dacite or silicic in that vicinity. andesite. There is remarkably little intergradation be- Peale Formation. tween these types. Dark quartz keratoph>rc flows and co-magmatic tuff-breccia Contacts between formations of the sequence, how- and tuff, probably ma- rine, characterized pink alkali ever, are generally gradational to some degree. Those by feldspar phenocrysts, dominate the lower that have not been demonstrated to be gradational, half of the Peale Formation. The upper half consists principally hence are conceivably unconformable, are the Taylor- of varicolored "ribbon" chert, slate, sandstone, Peale, Peale-Arlington, and Reeve-Robinson. Localh- tuffaceous and intraformational slump breccia. thickness there are breccias at the Goodhue-Reeve contact that Maximum approaches 2,500 feet. All are intermediate in chemical composition between the known manganese deposits in the northeast- ern Sierra in t\'pical Goodhue and Reeve breccias, suggesting a Nevada are the middle of the Peale For- mation. transition from one to the other; however, this is con- sidered only weak evidence for a gradational contact. Arlington Formation. Countless varieties of ande-

Clearl\-, there is a possibility that an unconformit\' sitic and dacitic tuff and tuff-breccia, water-laid and lies undiscovered above the Peale of the lower plate, well graded, and interbedded slate and minor volcanic and above the Goodhue of the upper plate, so that conglomerate-breccia comprise the Arlington Forma- there may be only partial equivalence of the Arlington tion, w hich is recognized onl_\' in the lower plate. In- with the Goodhue, Reeve and Robinson. tercalated breccia typical of the Reeve Formation in- Grizzly Fonnation. Found only on the upper dicates at least broad equivalence of the Arlington plate, the Grizzly Formation is a thin epiclastic unit with the Goodhue, Reeve, and Robinson succession. probably filling low places on the erosion surface on The apparent thickness is 8,000 feet. which it was deposited. It consists principally of black Goodhue Greenstone. The name Goodhue Green- laminated slaty shale and blocky siltstone, and quartz- stone is here applied to a distinctive pyroclastic brec- rich graywacke. It is generally abundantly interlaced cia which Diller (1908, p. 84 and p. 3) called Taylor with sills so that a true sedimentary thickness is dif- and referred to as lavas. The Goodhue, restricted to ficult to measure, but the maximum is probably close the upper plate, overlies the Peale Formation, whereas to 200 or 300 feet. Arkose occurring 20 miles south, the Taylor underlies the Peale. Andesite of the Taylor where Durrell (Durrell and Proctor, 1948, p. 171) is characterized by augite phenocrysts, whereas ande- first demonstrated the unconformity, may be nearl>- site or basalt of the Goodhue contains augite plus correlative. a relict second ferromagnesian phenocryst now rep- Sierra Buttes Formation. The name Sierra Buttes resented by magnesian chlorite or a serpentine mineral.

Formation is here proposed for Turner's quartz por- Derivation of the name is from the Goodhue home- phyry of the Sierra Buttes (1894, p. 483; 1896, p. 646; stead on Ward Creek, in the NE '/^ SW 'X SW 'X sec. ISO GfOUKiY OK NoRIHIRN CaIIKORMA Bill 190

14, T. 25 N., R. 11 F. The building is shown on near Peale, but with the .Arlington. Thus a Permian earlier topogrnphic maps, though not on the latest age for part of the Arlington is stronglx' suggested. (Kettle Rock, 1950). The type locality includes the 0\\ ing to the discovery of probable Devonian fos- east slope of Pcaie Ridge immediately west of Ward sils in limestone associated with volcanics in the foot-

Creek, and the type section is in the NE% sec. 22, T. hill belt (I,. D. Clerk, written communication, 1959), 25 N., R. 11 E. The thickness of the Goodhue Green- a Devonian age for the Sierra Buttes and Ta\'lor is stone is 1,500 feet. possible.

Reeve Fonnation. Keratoph\re breccia and tuff, Regional Relations of the Pyroclastic Sequence here and there fossiliferous, and minor fusulinid lime- stone and chert pebble conglomerate comprise the Most units of the pyroclastic sequence continue at Reeve Formation as redefined here. Diller apparently least 40 miles southeast along the west limb of the attempted to map the "sedimentary" tufif and lime- Ncvadan syncline to the North Yuba River and Mil- stone units in his Robinson Formation and the "igne- ton Reservoir. In some places all the .Milton Formation ous" breccia in his Reeve. More logical units are ob- of Turner (1897) consists of rock types characteristic tained b\' including ail rock t\pes intercalated in the of the Peale, perhaps the Goodhue, and certainly of breccia as a part of the Reeve, and restricting the the Reeve. Farther south the p\roclastic sequence Robinson to a distinctl\- different succession of vol- must be truncated by its bounding unconformity be- canogenic rocks which Diller also included in his cau.se at North Fork American River, about 50 miles

Robinson Formation. The Reeve is characterized b\' south of the Taylorsville area, Triassic(?) rests di- plagioclase phenocr\sts having major diameters of 10 rectly on the Shoo Fly Formation (Clark and others, to 20 millimeters, whereas those of the Robinson do 1962, p. B-18). Possibly correlative Pennsylvanian and not exceed 2 millimeters. The maximum apparent Pcrmian(r) strata in roof pendants 175 miles south are thickness is about 2,000 feet. nonvolcanic (Rinehart and others, 1959); these pen- dants define the east limb of the Nevadan syncline. Rob'uisoii Fonnatioii. As redefined above, the Rob- inson consists of andcsitic conglomerate-breccia, vol- Correlation onl\- on the grounds of lithlogic simi- caniclastic calcareous sandstone, slate, and minor lime- larity of the component formations of the pyroclastic stone. The maximum thickness is 700 feet. Fossils, sequence with specific formations outside the immedi- obtained from conglomeratic beds, are Paleozoic and ate outcrop belt ma\- well pro\e impossible, .•\lthough suggestive of a Permian age. the characteristic rock t\pes of the sequence are rcadil\- identified for 65 miles along strike, at the Age of the Pyroclastic Sequence northern end of the belt constituent fragments become

Only two horizons are dated b\' diagnostic faunas. so fine grained that identification becomes partl\- sub- The lower is at about the middle of the Peale Forma- jecti\e. If there has been appreciable telescoping along thrust faults farther west, individual lithic types prob- tion, and is at a localit\- reported b\' Diller (1908, p. 24). G. A. Cooper kindl\' studied the brachiopods abl\' do not persist across faults. However, it may from a new collection and determined them to be prove feasible to find equivalents of the pyroclastic Early Mississippian in age (written communication, sequence in a gross manner, assuming that the vol- 1965). canic episode responsible for the sequence was areall>' widespread. Perhaps the unfossiliferous Kanaka and The upper fossiliferous horizon is broadly in the Tightner Formations of Ferguson and Gannet (1932) middle of the Reeve Formation, which as re-defined and the Tightner of Chandra (1961, p. 12), which includes most of the localities Diller (1908, p. 27-28) contain abundant volanic material, are broadly equiva- included in his Robinson Formation. The most diag- lents. However, these formations are separated from nostic fossils are large fusulinids, which, however, are the outcrop belt of the pyroclastic sequence b\- a ma- too recry.stallized to identify specifically. Because of jor scrpcntine-bclt fault zone and are 60 miles south their large size, they are certainl\' Permian. Thompson of the Ta\lorsville area. Still farther south, in the and others (1946, p. II) state that Fusiiliiia eloiii^ata Foothill belt, Permian limestones included in the Cala- Shumard listed from the "Robinson locality" by Diller \eras Formation are associated with \()lcanic rocks is probabl\- I'ara^usiiliiia. (Clark, A Permian age was formerly conceded for the 1964). Taylor (specifically for the augite porphyrite of Tur- Correlation with late Paleozoic rocks outside the ner) because a mold of Hclicoprioii sicrreiisis discov- Sierra Nevada cannot be attempted even on a gross

ered in Pleistocene till was supposedly derived from lithologic basis because of facies changes, and is there- Turner's augite pf)rphyrire (VVheeler, 1939, p. 107). fore dependent on paleontologic dating. The nearest

However, in the glacial basin from which the till must correlatives in all directions except to the southeast have been derived. Turner included Peale and prob- contain appreciable volcanic constituents and gener- ably Arlington rock t\pcs in his augite porphyrite. ally are as sparsely fossiliferous as the late Paleozoic Moreover, a pctrographic description of the matrix of the Taylorsville area. For the interested reader, the made for Wheeler compares neither with the Taylor more recent pertinent papers are by Coogan (1960), 1966 McMath: Tayi.orsvii.ik Arka, Northern Sikrra Nkvada 181

Albers and Robertson (1961), and Silberling and Swearinger, but flowage during folding, incomplete Roberts (1962). resolution of structure, and other factors already cited Triassic Sequence preclude meaningful measurements. Rocks of the Triassic System, consisting of the t\pe The Hosselkus consists largel\' of dark gray or black Hosselivus Limestone and the Swearinger Formation, aphanitic limestone which weathers light gra\', and is are exposed in the Tayiorsvilie area only on the frontal thinly bedded to laminated where bedding is visible. margin of the upper plate of the Tayiorsvilie thrust. Beds of calcarenite, commonly containing rounded The\' are relatively thin, rather abundantly fossilif- quartz grains, crop out near both margins. Locally erous compared to subjacent strata, and in contrast abundant white splotches generally represent sheared- out and recrystallized fossils. to all other systems in the area, quite free of identified volcanic debris. Their setting was more that of the Black, calcareous, laminated hornfels having little quiescent shelf than of the mobile eugeosyncline that relict fissility characterizes the Swearinger Formation. both preceded and followed their deposition. The\- Thin black argillaceous limestone, quartzose sandstone rest with angular unconformity on both known and beds, two of which are imperfectly graded and indi- questionable Permian strata, and their upper boundary cate top to the east, and a little conglomerate are also is the trace of the Tayiorsvilie thrust. Together with present. the Tayiorsvilie thrust, they are complexly folded, lo- Age assignments for the two formations have not cally involuted, and are thoroughl>- hornfelsed where changed appreciably from those given b\' 'Hyatt an adjacent granitic pluton has intruded them. (1892, p. 399): Late Triassic and specifically Xorian.

Westermann ( 1962, p. 753), the Diller (1892) and Hyatt (1892) named the two .\ccording to G. E. G. of the bearing Monotis siihcirciiUnh Gabb is iithic units the Hosselkus Limestone and the Swear- age beds Westermann further postulates inger Slate, and Hyatt dated them as Late Triassic. probably late Norian. in shallow water, perhaps attached The Swearinger dips under the Hosselkus and was that Monotis lived therefore presumed to be the older, but Diller (1908, to seaweeds. Correlation of at least the Movotis beds of the p. 33) subsequently concluded that the Swearinger is part of the Brock Shale of Shasta the younger and that both are therefore overturned.' Swearinger with County seems well established (Smith, 1927). Other On the basis of sedimentary structures indicating Late Triassic or supposcdl\- Late Triassic formations the Swearinger is not overturned, and a faulted contact that may correlate with either or both the Hosselkus between the Triassic and Permian(?), McMath (//; and Swearinger are the Pit Formation and "Hosselkus" Reeside and others, 1957, p. 1470) reversed the se- Limestone of Shasta County (Diller, 1906; Smith, quence to agree with Diller's original interpretation. 1927; Sanborn, 1960; Albers and Robertson, 1961), the Subsequent field stud\' showed the sedimentary struc- Cedar Formation, which is a few miles west of the tures to be contained in involuted beds of the Robin- Tayiorsvilie area (Smith, 1894; Diller, 1895), limestone son Formation faulted into juxtaposition with the on the North Fork American River, 50 miles to the Swearinger. The faulted contact between the Robinson south (Clark and others, 1962), and possibly part of and the Hosselkus must be an example of an uncon- the Milton Formation 30 miles south in the Downie- formity "unglued" by local flexure folding—an incip- ville quadrangle (Turner, 1897). ient decollement—because a definitive outcrop later It is conceivable that the boundary between the discovered outside the zone of intense folding showed Triassic and Jurassic sequences is exposed but not yet a depositional contact. History has repeated itself: the recognized. The upper part of the Swearinger con- sequence is once again reversed. May this Triassic tains considerable hornfelsed feldspathic sandstone section forever rest undisturbed upside down! that could readih- mark the base of the Jurassic. Al- The contact between the Hosselkus and Swearinger ternatively, if the uppermost part of the Triassic se- is gradational. Representative thicknesses are about 200 quence were volcanic, it is possible that a horizon in feet for the Hosselkus and perhaps 900 feet for the the oldest known Jurassic formation, the Lilac, marks ^ Inversion of the apparent sequence was based on correlation with an the boundary. A third possibility, at least as likely as Upper Triassic section, also described by Diller (1906), in Shasta County about 100 miles to the northwest. There, two shale forma- the first, is that the Triassic-Jurassic boundary may tions lithologically comparable to the Swearinger lie both above and lie hornfels, which underlies the more obvi- below a limestone to which J. P. Smith (1894, p. 604-609) applied in dark the name "Hosselkus." He correlated the limestone and the under- ously volcanic part of the Kettle Formation at its lying shale unit—the Pit Formation—with respectively the type Hosselkus and the type Swearinger. The apparent sequence of the eastern border. two sections is the same. This correlation was discredited when Smith (1898. p. 778) subsequently correlated the shale above the Jurassic Sequence "Hosselkus"—the Brock Shale—with the Swearinger. The faunal tie of the Swearinger witli the Brock proved to be closer than with the The marine Jurassic section exposed principally on Pit Formation. But was retained for the lime- the name Hosselkus has long been an outstanding geologic at- stone. And this was done despite the fact that the t\-pe Hosselkus .Mount Jura was dated so inexactly that Diller subsequently felt free to shift it traction of the Tayiorsvilie region. Like the late Paleo- from above the Swearinger to below. Thus Diller inverted the apparent sequence of the type Hosselkus and Swearinger to make zoic sequence, it is volcanogenic and it is structurally them agree with the supposedly equivalent Shasta County section. He did not question the initial correlation of the type Hosselkus and stratigraphicall\' ncarl\- as complex, but on a with the Shasta County limestone. Correlation of these two lime- smaller scale. "It is structurally so complex that a stones is not established even today according to N. J. Silberling (oral communication, 1965). Nonetheless, Diller was correct in placed at its base b\- one of the t\\ o chief reversing the sequence. formation 182 Geology of Northkrn California Bull. 190 authorities is placed by the other authority at its Jurassic stction on top" (Reed, 1943, p. 106). Owing to the relative abun- dance of fossils and of distinctive rock t\pes, the prin- cipal investigators, J. S. Diller (1892, 1908) aided by .\lpheus Hyatt (1892), and C. H. Crickmay (1933) were induced to erect 14 formations; two more arc recognized in the present study. One is an unnamed dacite tuff-breccia former!)- regarded as intrusive, and the other, the Kettle Formation (Kettle Meta-andesite of Diller), was regarded by Diller (1908, p. 84-85) as Carboniferous.

Crickmay published only a tabular summary of his restudy of the Jurassic sections. In such a brief treat- ment of his results, he was not able to marshal the evidence for his changes, hence some of them were not readily accepted. Though the present study of the

Jurassic is incomplete, and revision will not be docu- mented, an advance sumn>ar\' in support of Crickmav's stratigraphic succession seems warranted. The Kettle Formation and the overlying Trail For- mation of Diller (which is equivalent to Crickmay's Lucky S, Trail, and possibly Combe Formations) con- stitute the normall\' dipping east limb of the Nevadan syncline. Although the Kettle is cut out on the north- east by a granitic pluton, the total thickness of the two formations is more than 15,000 feet. The sections on Mount Jura are equivalent to the Kettle in whole or in part, are similarly overlain by Diller's Trail Forma- tion (but mapped by Diller in his Foreman Forma- tion), and constitute the western and overturned limb of the Nevadan s\ncline. Thc\' are repeated by a complicated fault system that appears to be essentialh a folded gravity-slide thrust. The upper plate of the Taylorsville thrust largely conceals the synclinal axis, and completely conceals the lower stratigraphic boundar\' of the Mount Jura sections.

Crickmay (1933, p. 897) stated that his oldest Juras- sic formation, the Lilac, rests on Middle Triassic vol- canics. Because Upper Triassic rocks were presumably derived b\- thrusting from the same limb of the Neva- dan s\'ncline as the Mount Jura sections, it is sug- gested that the volcanics in question are even older Jurassic, and that Upper Triassic rocks underlie the volcanics but are concealed by the upper plate of the Taylorsville thrust.

The Mount Jura sequence is outlined in the table below. Thicknesses are generally those listed by Crick- may. The Combe Formation of Crickmay has not yet

been identified. Its age is controversial because Crick-

may dated it as younger than the Nevadan orogeny.

The Kettle Formation is equivalent to the Cooks Canyon and earlier formations, and includes the same gross lithic types. A dacite tuff-breccia in the middle of the formation is assumed to be the same as the unnamed dacite tuff-breccia of the Mount Jura sec- tion, and the block diagram has been constructed on

this assumption. The total thickness of the Kettle is 1966 McMath: Tayi.orsvili.f, Area, Northern Sierra Nevada 183

ously of time and advice, Cordell Durrell and John C. Graduate Faculty of the University of California, Los Croweil deserve special mention. Part of the paper is Angeles. The Continental Oil Company provided gen- condensed from a Ph.D. dissertation submitted to the erous aid in the preparation of the manuscript.

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Botemon, P. C, Clark, I. D., Huber, N. K., Moore, J. G., and Rinehorl, California: U.S. Geol. Survey Geol. Atlas, Folio 66.

C. D., 1963, The Sierra Nevada batholith—a synthesis of recent work Reed, R. D., 1943, California's record in the geologic history of the 414-D, across the central part: U.S. Geol. Survey Prof. Paper p. world: California Div. Mines Bull. 118, p. 99-118. D41-D46. Reeside, J. B., Jr., chm., and others, 1957, Correlation of the Triossic Chondra, D. K., 1961, Geology and mineral deposits of the Colfax and formations of North America exclusive of Canada, with a section on ForesthitI quadrangles, California: California Div. Mines and Geology Correlation of continental Triossic sediments by vertebrate fossils, Spec. Rept. 67, 50 p. by E. H. Colbert and J. T, Gregory: Geol. Soc. America Bull., v. 68,

Clark, L. D'., 1960, Foothills fault system, western Sierra Nevada, Cali- p. 1451-1513. v. fornia: Geol. Soc. America Bull., 71, no. 4, p. 483-496. Rinehort, C. D., Ross, D. C, and Huber, N. K., 1959, Paleozoic and 1964, Strotigraphy and structure of port of the western Sierra Mesozoic fossils in a thick stratigraphic section in the eastern Sierra Nevada metamorphic belt, California: U.S. Geol. Survey Prof. Paper Nevada, California: Geol. Soc. America Bull., v. 70, no. 7, p. 941-945.

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northern Sierra Nevada, California: U.S. Geol. Survey Prof. Paper Silberling, N. J„ ond Roberts, R. J., 1962, Pre-Tertiary strotigraphy and 450-B, art. 6, p. B15-B19. structure of northwestern Nevodo: Geol. Soc. America Spec. Paper Coogon, A. H., 1960, Stratigraphy and paleontology of the Permian 72, 58 p. Univ. Nosoni and Dekkos Formations (Bollibokka Group): California Sloss, L. L., Krumbein, W. C, and Dapples, E. C, 1949, Integrated Sci., v. Pubs. Geol. 36, no. 5, p. 243-316. facies analysis, in Longwell, C. R., chm.. Sedimentary facies in

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Mineralogist, v. 45, nos. 3-4, p. 454-455. Smith, J. P., 1894, The metamorphic series of Shasta County, California: 1961, Some recent work on the lower grades of metomorphism: Jour. Geology, v. 2, p. 588-612. Austrolion Jour. Sci. v. 24, no. 5, p. 203-215. 1898, Geographic relations of the Trios of California: Jour. Crickmay, C. H., 1933, Mount Jura investigation: Geol. Soc. America Geology, v. 6, p. 776-786.

Bull., V. 44, no. 5, p. 895-926. 1927, Upper Triossic marine invertebrote faunas of North America: Diller, J. S., 1892, Geology of the Toylorsville regionregi( of Califo U.S. Geol. Survey Prof. Paper 141, 262 p. Geol. Soc. America Bull., v. 369-394. 3, p. Taliaferro, N. L., 1942, Geologic history and correlation of the Jurassic 1895, Description of the Lassen Peak quad angle, California: U.S. of southwestern Oregon and California: Geol. Soc. America Bull., Geol. Survey Geol. Atlas, Folio 15. V. 53, no. 1, p. 71-112.

1906, Description of the Redding quadn ngle, California: U.S. Thompson, M. L., Wheeler, H. E., ond Hozzord, J. C, 1946, Permian Geol. Survey Geol. Atlas, Folio 138. fusulrnids of California: Geol. Soc. America Mem. 17, 77 p.

1908, Geology of the Toylorsville region, California: U.S. Geol. Turner, H. W., 1894, Rocks of the Sierra Nevada: U.S. Geol. Survey Survey Bull. 353, 128 p. Ann. Rept. 14, p. 435-495.

Durrell, Cordell, and Proctor, P. D., 1948 deposits near Lake 1896, Further contributions to the geology of the Sierra Nevada: Howley and Spencer Lakes, Sierra County, California California U.S. Geol. Survey Ann. Rept. 17, p. 521-762.

Div. Mines Bull. 129, pt. L, 165-192. p. 1897, Description of the Downieville quadrangle California: U.S. Ferguson, H. G., and GanneH, R. W., 1932, Gold quartz veins of the Geol. Survey Geol. Atlas, Folio 37. Alleghany district, California: U.S. Geol. Survey Prof. Paper 172 Westermann, G. E. G., 1962, Succession and variation of Monofis and 139 p. the associated fauna in the Norian Pine River Bridge section, British Hyatt, Alpheus, 1892, Jura and Trias at Toylorsville, California: Geol Columbia (Triossic, pelecypodo): Jour. Paleontology, v. 36, no. 4, Soc. America Bull, v. 3, p. 395-412. p. 745-792.

Imlay, R. W., 1952, Correlation of the Jurassic formations of North Wheeler, H. E., 1939, He/icoprion in the Anthrocolithic (late Paleozoic) America, ex ilusive of Canada: Geol. Soc. America Bull., v. 63, no. 9, of Nevada and California, and its strotigrophic significonce: Jour. p. 953-992. Paleontology, v. 13, no. 1, p. 103-114. S2 5 ; I III

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