Late Cretaceous (Santonian-Campanian) stratigraphy of the northern Sacramento Valley, California DcT^rf {Department of Geology, University of California at Davis, Davis, California 95616 rElbK L). WAKL) J ABSTRACT INTRODUCTION METHODS The Upper Cretaceous (Coniacian-lower Thick accumulations of Upper Cretaceous Strata of the Chico Formation dip gently to Campanian) Chico Formation of the north- sedimentary deposits are found on the western, the southwest. Sections were mejisured using eastern Sacramento Valley, California, includes northern, and eastern margins of the Great Val- either tape and compass or Jacob's staff. In some three newly defined members at the type local- ley of California (Fig. 1). The search for oil and areas, outcrop data were plotted on U.S. Geo- ity: (1) cobble conglomerate of the basal Pon- gas in northern California, as well as interest in logical Survey topographic quadrangles and derosa Way Member, (2) coarse-grained con- the processes of sedimentation in fore-arc re- stratigraphie columns were determined trigo- glomeratic sandstone of the overlying Musty gimes, has made the Great Valley seque nce, ex- nometrically. Paleontologic collections of mac- Buck Member, and (3) fine-grained silty sand- posed along the west side of the Sacramento rofossils were made during the measuring of stone of the uppermost Ten Mile Member. Valley, probably the best-studied fore-arc de- sections. Minor offset of bedding was observed Other outcrops of the Chico Formation exhibit posit in the world (Ojakangas, 1968; Dickinson, on more southerly exposures of the Chico For- the same three members plus an additional unit, 1971; Ingersoll, 1978, 1979). These workers in- mation, and such structural modification be- the Kingsley Cave Member, composed of mud- terpreted strata of the Great Valley sequence to comes more prominent farther north. These stone. The Chico Formation resulted from a be deep-marine slope and basin-plain deposits. disruptions are easily recognized and the offset transgression onto the Sierra Nevada basement Many of these rocks contain an abundant micro- can be compensated for, except on Antelope in the Late Cretaceous. The Kingsley Cave fauna that has been utilized for correlation of Creek, where severe folding has occurred at Member was deposited locally in quiet-water diverse lithologic types (Goudkoff, 1945; some localities. Such areas were treated as iso- conditions below wave base in an intrashelf Douglas, 1969). basin and may represent a marine connection In contrast, Upper Cretaceous strata on the with regions farther northeast. eastern side of the Great Valley, including lateral Macrofossils allow correlation of the Chico equivalents of the gas-producing strata of the Formation with marine deposits of the Great Sacramento Valley, have received only minimal Valley sequence exposed along the western attention. This is due, in part, to the relative Sacramento Valley. Strata of the Guinda and paucity of a microfauna, as well as to the re- Forbes Formations represent submarine fan- moteness of most exposures. Little is known, channel to outer-fan deposits of a shallowing consequently, of the depositional environments Late Cretaceous fore-arc basin. The Santonian of these proximal facies, and biostratigraphic re- Guinda Formation, a massive sand unit depos- lations have not been defined in most sections. ited in fan-channel to distal-fan turbidite envi- In this paper, we describe new sections with ronments, is correlative with the Musty Buck paleontologic collections from the Chic» For- and Kingsley Cave Members of the Chico For- mation in Tehama and Butte Counties. Meg- mation. The Dobbins Shale Member of the afossils now allow correlation between both Forbes Formation, a widespread mudstone unit margins of the Great Valley, thus uniting out- of hemipelagic outer-fan to basin-plain deposits, crops of this depositional system into a compre- is correlative with the Kingsley Cave Member of hensive littoral to bathyal sedimentologic the Chico Formation and reflects a Santonian framework. transgressive event in the northern fore-arc basin. Mudstones and turbidites of the middle of the Forbes Formation are equivalent to the shallow-marine strata of the Ten Mile Member Figure 1. Distribution of Map Area of the Chico Formation. Correlatives of younger Upper Cretaceous outcrop Forbes Formation strata have not been posi- along the margins of the tively identified from the Chico Creek region Sacramento Valley. Stippled but may be represented by unfossiliferous, cross- area represents outcrop of bedded, coarse-grained sandstones found above the Chico Formation. sections of the Chico Formation. Geological Society of America Bulletin, v. 95, p. 618-627, 11 figs., May 1984. 618 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/5/618/3445089/i0016-7606-95-5-618.pdf by guest on 26 September 2021 CRETACEOUS STRATIGRAPHY, SACRAMENTO VALLEY, CALIFORNIA 619 of Chico "should be abandoned as a group or which traverses the higher elevations of the field series name, but that it might usefully be re- region. The member unconformably overlies tained as a formational name for the rocks at pre-Cretaceous metamorphic rocks, this contact and near the type locality on Chico Creek." Re- being obscured in most areas by a cover of ero- cent workers have, as a rule, followed this sional debris. In the extreme northeast corner of suggestion. sec. 1, T. 23 N., R. 2 E., exposures on Big Chico The overlying and extensive Tuscan Forma- Creek in the vicinity of the Ponderosa Way tion of Pliocene age, and locally the Eocene Bridge are thus arbitrarily defined as the lower- Lovejoy Basalt, limit exposures of the Chico boundary stratotype for the Ponderosa Way Formation to narrow outcrops along only the Member. The member is approximately 200 m most deeply incised drainages of the Lassen thick on Big Chico Creek, with the upper- foothills region (Fig. 2). The first investigations boundary stratotype located in the center of sec. of Chico strata were those of Trask (1856), who 1, T. 23 N., R. 2 E. The member is well exposed described ammonites from the Cretaceous de- along Butte Creek, 8 km to the southeast, both posits exposed along Big Chico Creek. Diller above and below the Helltown Bridge crossing; (1889, 1895) mapped additional Chico Forma- here the stratigraphic thickness is somewhat less tion outcrops on Deer, Mill, and Antelope than on Big Chico Creek, —180 m. Creeks and noted that Cretaceous fossils had The lower part of the member consists of been found in those areas. However, due to their cobble conglomerate with local sand lenses remoteness, these outcrops have never been stra- (Fig. 4). Cobbles are poorly sorted and moder- tigraphically measured or collected. ately well rounded, and they range in size from 6 North of the Chico region, Cretaceous stratig- to 20 cm in diameter, with some as much as 70 raphy and molluscan occurrences in the vicin- cm. Clasts are primarily matrix-supported and ity of Redding, California, were summarized by consist of chert, metavolcanics, quartzite, schist, Popenoe (1943), who recognized six lithologic and slate, suggesting that they were derived from members. On the basis of macrofossils, he the underlying metamorphics. Granitic clasts equated the youngest of these deposits with the from nearby Mesozoic Sierran intrusives are ab- Figure 2. Outcrops of the Upper Creta- base of the section exposed along Big Chico sent. Clasts are embedded in a medium- to ceous Chico Formation, Tehama and Butte Creek. Matsumoto (1959-1960, p. 4) proposed coarse-grained arkosic sandstone matrix. Rare Counties, California. the name Redding Formation for these strata, trough cross-stratification occurs within the whereas Jones and others (1978) suggested that sandstone lenses, and there are indistinct, normal lated sections. Additionally, infilling of topo- at least some of Popenoe's (1943) Members grading and cobble imbrication in some graphic lows in the Cretaceous surface by I-VI were of formational rank. sections. younger volcanics results in isolation of some of The type locality for the Chico Formation, The upper portions of the member become the outcrops from the rest of the section. These Big Chico Creek, was established by Taff and increasingly sandy through interbedding of exposures were placed into a composite section others (1940) and has been the principal field sandstone lenses within the massive cobble con- based on the structural attitudes of the sediments area for biostratigraphic and sedimentologic glomerate. On Big Chico and Butte Creeks, the bracketing them. By necessity, then, some seg- studies. Saul (1959,1961) outlined the sedimen- transition to the overlying Musty Buck Member ments of the stratigraphie sections are incom- tologic units and the molluscan succession ob- is gradational at the localities noted above. Ma- plete, resulting in local truncation of lithologie served at the type locality. She recognized three rine fossils (ammonites and other mollusks) and faunal ranges. distinctive lithologies and informally designated have been reported from only the uppermost them the Ponderosa Way Member, the Musty exposures of the Ponderosa Way Member on STRATIGRAPHY Buck Member, and the Ten Mile Member. This Big Chico Creek (Matsumoto, 1959-1960). paper follows her usage in formal presentation. Exposures of the member along Butte Creek East Side: Lithostratigraphy Further examination of Chico Formation out- lack a normal marine macrofauna. crop along Deer, Mill, and Antelope Creeks has In the 125 years since the introduction of the shown the presence of these same three mem- Musty Buck Member name Chico for Upper Cretaceous sedimentary bers, plus an additional unit, herein designated deposits of the Pacific Coast of North America, the Kingsley Cave Member. Stratigraphic rela- The Musty Buck Member rests conformably the term has been employed in a variety of litho- tions of Chico Formation outcrops are summar- on the underlying Ponderosa Way Member on logie contexts. Cretaceous strata from California, ized in Figure 3. Big Chico and Butte Creeks.