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The contact betlJeen the St. Charles and Garden City Formatlons In the Bear River Range, Bannock thrust sheet In northern Utah and southeastern Idaho. Is 8 diachronous dlsconformlty within the Lower Ordovician. Middle or, pos s lbly, upper Cordylodu8 proavu8 Zone (Clavohamulu8 elongatuB to, poss1bly. £:.. hlntzel Subzones) conodonts indicate that lowermost Canadian Series (Lower Ordovician In North American usage) strata equ! valent to the Mls81squoia Zone and, pt)8s1bly. lower Symphysurina Zone (trilobites) are present In the upper part of the St . Charles Formation. The boundary between the T'rempealeslJ;I \ Stage and Canadian Series (Cambrian-ordovlcian boundary) is located within the upper part of the St. Charles Formation. Basal beds of the Garden City Formation contain conodonts representing Fauna B (upper part) in two Idaho sections and Fauna C at two Utah localities . At least 12 m (40 feet) of erosional relief was Ipresent address: New York State Geological Survey, State Education Tl!:.partment I Albany. New York 12230 developed on the dolos t ones of the St. Cha rles Forma tto n prlo r to deposition of the Ga rden City Fo rmation. TrUoblte Zone A of Ross from the lower part of the Carden City Fomatio n co rrelates wt th the upper part of the Symphysurina Zone (.!.. bulbosa Subzone ) and corres ponds in its lower and upper parts, respectlvely , with uppe r Fauna " and Fauna C (conodonts) . Conodont Fauna C occurs with trilobite s of upper Zone A through Zone C of Ross. Conodont Fauna n a nd trilobite Zone D have a lowest occurrence a t approximately the same s tratigraphic level, although the lower pa r t of trilobite Zone D of Ross lles within the upper part of conodont Pauna C. Conodont elements from the lower part of the Carden City Formation have a color alte rotion index ( CAl) of low 4, whereas the strongly corrode d specimens throughout the St . Charles dolos tones have a n apparent CAl of 6.5, although associated rarer elements ha ve a CAI-4(low) . The color alteration index de veloped for conodonts from limes tones may not apply to samples from dolos tones that have ha d their primary organic mat ter reduced by leaching during dolomitization . INTRODUCTION The contact be tween the St . Charles and Carden City Formations i n the Bear River Range of northern Utah and southeastern Idaho ls a striking lithologic break in a ralogeosynclinal carbonate plat form sequence. Massive dolostones of the St . Charles Formation are succeeded by llmestones of the Carden City Formation (Richardson , 1913) containing a trilobite and brachiopod sequence (Ross, 1949, 1951, 1953, 1968) that has come to be regarded as a comparative standard for Early and early HictrUe Ordo vician fAunRl co rre l a tions 1n the No r th Ame rtca n Faunal Province. Shelly fa unAS ha ve not he en reco rded from the uppermost pa r t o f the St. Charles Format to n, a nd t he re has been no a vatlable mac rofoss Il evtdence to suppor: c l a t ms that the Ca rden City Formation conformably (Ros s , 1949, 1951) or disconfo rruably (Willia ms, 1948 ; Richardson, 1913) overlles the St. Charles Pormat t o n . The :> r esent study wa s i ni tiated t o provide a conodont zona tion fo r the upper part of the St. Cha rles Formation and lower part of the Carde n City Formation and to evaluate the conodont evidence for the possiblity t hat the contact between the format tons represents a hiatus . The conodont zonation also provIdes a partial basis for compa ris on of Ross' (1949, 1951) trllobite Zone A through lowest Zone D with other known conodont a nd trilobite sequences. The uppe r 60-75 m (200250 feet) of the St . Chsrles and the lower 50 .. (165 feet) of the Carden City FormatIons were sampled for conodonts a l: f our localities in the western limb of the Logan Pea k syncline of the Bannock thrust sheet (Richard s a nd Mansfield, 1912 ; Williams. 1948), Bear River Range. northern Utah a nd southeastern Idaho (fig . 1) . These sections a re at or near Ross' (1949, 1951) localit1es a t Franklin 'Rasln and HIllyard Canyon, Franklin County, Idaho , a nd Blacksmith Fork Canyon a nd Creen Canyon, Cache County, Utah . Associated trilobites, petrograrhic features, and regiona l st r atigraphy are presently under study by H. E. Taylor a nd J . E. Repetsk1. The results reported here apply to the central portion of the Bear River Range, Utah-Idaho , and mayor may not be applIcable to the lowest bed s of the Carden City Formatton in other 10ca lLtLes Ln the northern Creat Ba B Ln and central Rocky r·founta Ln regions. The contact between the St . Cha rles and Garden City Forll"3tlons t a an abrupt change frolll massively weathering , cliffformIng, locally sucrosLc dolostones below, into thinto medium-hedded 1 Lmestones above. An interval of thin-bedded, crossbec1ded, fIne-grained, brownish-grey dolostone wt th dolomitized inl ~aclast8 a nd sparse quartz sanc1 is eeneratly present above the massive dolostones. This thin-bedded dolostone untt is dlssltdlar to do lOB tones in the St. Charles Formation and wss aBsie-ned to the Carden City Formation in the course of field work. The thin-hedded dolostone unit in the basal Carden City Porm.tlon 1. O . l~ m (0.5 feet) thlck and 6.15 m (20 . 5 feet) thlck . respectively, 1n the well-exposed Franklin Basin anli Blacksmith Fork sections . This dolostone untt is 14.8 m (49.) feet) thick at Hillyard Canyon but 1s partially repeated by a small fault along the measured traverse. Some fault-related dolomitization may have taken place in the lowermost Garden Clty Formation at Hillyard Canyon, but thin-bedded dolostone at Franklin Basin a nd Blacksmith Pork Canyon is not associated wlth faulting and may be of early diagenetic orlgin . Finally, considerable tectonic shearing has taken place in the upper St. Charles Formation in the Creen Canyon section and appears to be related to an easterly dipping fault zone . However, at least 2.77 m (9.25 fe l'!t ) of thin-bedded dolosto ne is present at the base of the Carden City Formation at Green Canyon. CONODONT BIOSTRATtGRAPIIY Remarks.--When present conodonts a re spa rsely represented 1n the upper part of the St. Charles Formation (commonly less than elemen t s per kllogr am) a nd provlde a satisfactorIly resolved correlation only for the uppermost m (20 feet) of the formatIon. DLverse and ahl1ndant conodont faunas (greater than 500 elements per kilogram) c ha racterize the lowermost dolostone and lower limestones of the Ca rden City Formation. The observed fau nal sequence and the local range zones of selected conodont taxa are recorded in f Lgures 2-5 . !!... Charles Formation.--Clavohamulus elongatus Hiller, 1969 (_£. elongatus and .£.. primittvus Miller, 1969), with Cordylodus prodvus Haller, 1959, ~ formo i! .. !..), Eoconodontus notchpeakensis (Hiller, L969), and Teridontus nakamurai (Nogami, 1967) occur in the uppermost St . Charles Formation at Franklin Rasi n (upper 7.8 m; 26 feet) and at Rillyarc1 Canyon (upper 3 .3 m; 11 feet) in association with Cordylodus intermedius Furnish, 1938, s. f. This assemhlage indicates correlation with the uppermost Notch Peak Formation, Ibex area, west-central Utah (Miller, 1969, 1978, 1980; Rintze and others, 1980) . A comparable fauna is present in sample 03l8S-CO from at least 10 . 8 m (36 feet) below the top of the St . Charles POnDation at Green Canyon. A highly resolved biostratigraphic correlation of conodont faunas from the uppermost part of the St. Charles .·ormation is not possible, although an interval in the upper part of the Cordylodu8 proavus Zone (Clavohamulua elongatus through, possibly, ~. hlntzei Subzonea) is indicated for these Early Ordovician (Canadian) faun as . t:lavohsmulus elongatll8 has been r epo rted t o be restricted to the .£ .. e l ongatus Suhzone (middle Co rdylodus proBvus Zone) in the Ibex area , whe r eas Cordylodus inte rmedius .! .. ~. fLrst a ppea r s in the s uperjacent IUrs utodontus simplex Subzone in that a r ea (Miller, 1978, 19RO; Hi ntze and ochers. 1980) . However , Miller ( 1969) has also noted chat the loca l range zone of f .. elongatus overlaps that of .£. hintzei, the eponymous species of the uppermost s ubzone of the Cordy loduB proavus Zone , whereas f.. i"termed ius . ! .. .!.. r a nge s lower in the Cordylodu9 proavus Zone . The biostratigraphic corre l a tion of the uppermost part of the St . Charles Formation at Creen Canyon, Hillyard Canyon, and Franklin Basin w1th the Cl a vohalDulus elongatus Subzone(?) is proposed in this r e por t because conodonts s uch 8S Utahconus spp., JU r sutodontus simplex . Clavohamulus hintzel, a nd Monocostodus sevierensis (Miller, 1969), which first appea r in the upper pa rt of the Co r dylodus proavus Zone, are not prese nt in the rich co llec tions (more than 1 , 000 e lement s ) f rom this interval. This admittedly negative evidence does not rule out the possibility that C. elongatus Subzone(?) of the uppermost part of the St. Charles 'Formation may actually include upper pa rts of the Cordylodus proavus Zone. Samples from the uppermost St. Charles Formation at Rlacksmith Fork Canyon yielded f.. intermedius .!:.!.' In the absence of Clavohamulus elongatus. 'nle uppermost St. Charles at this section is lowest Cansdian . Ilnd there is no evidence fo r or against a tentative correlation with the middle Cordylodus proavus Zone . Conodont-based correlation of lower beds in the upper St. Cha rles Formation a r e much more probl ematical . The local range zones of primitive cordylodans extends 13.5 m (45 feet) (sample D3102-CO ) a nd 18 m (60 Eeet) (salUp l e D3182-CO) below the top oE the St . Cha rles at Blacksmlth Fork and Gr een Canyons, rcspect l.ve l y, and indicate that these po r tions of the section represent the Cordyl odus proavus Zone of the uppermost Trempea leauan Stage and lowest Canadian Series . Eoconodontus notchpeakensis in samples D3101-CO and D3 179-CO Er olU 45 . 6 m (152 feet) and approximately 66 m (220 feet) , res pectively

u.s. geological survey open file report/open-file report

Conondont biostratigraphy and thermal color alteration indices of the upper St. Charles and lower Garden City formations, Bear River Range, northern Utah and southeastern Idaho