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New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/23 Jurassic strata in northeastern New Mexico Charles J. Mankin, 1972, pp. 91-97 in: East-Central New Mexico, Kelley, V. C.; Trauger, F. D.; [eds.], New Mexico Geological Society 23rd Annual Fall Field Conference Guidebook, 236 p. This is one of many related papers that were included in the 1972 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. These books have set the national standard for geologic guidebooks and are an essential geologic reference for anyone working in or around New Mexico. 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One printed copy of any materials from the NMGS website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires explicit permission. This page is intentionally left blank to maintain order of facing pages. JURASSIC STRATA IN NORTHEASTERN NEW MEXICO by CHARLES J. MANIKIN Director, Oklahoma Geological Survey Norman Oklahoma INTRODUCTION this error by using the original spelling. But because of wide- spread use of the spelling "Exeter," and because the Exter The Jurassic System in northeastern New Mexico is repre- Post Office is no longer in existence, Lees spelling has been sented by a rock sequence consisting predominantly of sand- accepted. stone, siltstone, mudstone, and shale with a few thin, discon- Dobrovolny and Summerson (1946) used the term "Win- tinuous layers of impure limestone. This sequence is divided gate(?) sandstone" for this rock unit in describing the surface into the Exeter Sandstone at the base, overlain by the Mor- geology and Mesozoic stratigraphy of a part of northwestern rison Formation. Locally, an algal limestone is present be- Quay County, New Mexico. tween the Exeter and Morrison (see fig. "I). Bachman ("1953), in a report on the geology of a portion of The name "Bell Ranch Formation" as defined by Griggs and northwestern Mora County, New Mexico, applied the name Read (1959, p. 2006) for a thin-bedded sequence of gray sand- "Ocate sandstone" to this rock unit. stone and brownish red siltstone at the top of the Exeter in In a subsequent study of the distribution of this sandstone the Tucumcari-Sabinoso area has not been used in this paper. unit in northeastern New Mexico the writer (Mankin, 1958) The writer concurs with Traugers observations (Groundwater, concluded that the "Wingate(?) sandstone" of Dobrovolny and this guidebook) questioning the need for this separation. The Summerson and the "Ocate sandstone" of Bachman were all a sequence may be locally distinctive but it is doubtfully a part of the same lithosome. The term "Exeter" was applied to lithosome continuous enough to deserve formational rank. this unit throughout its aereal extent in northeastern New A thin, widespread red chert layer in the lower part of the Mexico in recognition of the nomenclatural priority estab- Morrison Formation serves as an important stratigraphic lished by Lee (1902). marker in this region. At a few localities gypsum beds are also In an earlier study Heaton (1939) correlated the Exeter present in the lower part of the Morrison. At most places the Sandstone with the Entrada Formation of the Four Corners rock record appears to represent a time of continuous con- area (G illuly Reeside, 1927, p. 76) and used the latter name tinental sedimentation; no evidence has been presented to sug- for the Exeter. Griggs and Read (1959), in a review of the gest a widespread hiatus. stratigraphic nomenclature of the Tucumcari-Sabinoso area of The Jurassic strata overlie rocks of Triassic age, separated by northeastern New Mexico, used the term "Entrada sandstone" a regional disconformity with local angular discordance. These without reference to the term "Exeter" or its priority in this strata are in turn overlain in most places by rocks assigned to region. Subsequent workers have used the term "Entrada" the Cretaceous System. The J urassic-Cretaceous boundary is (e.g., Trauger and Bushman, 1964) without further comment. interpreted as a regional disconformity, but in several localities The writer has used the term "Exeter" in preference to the the evidence supporting the discontinuity is tenuous. more widely accepted usage of "Entrada" to emphasize the Although paleontological evidence to support a Jurassic age fact that no strong correlation of this sandstone unit in north- assignment for these strata is sparse and the precise age correla- eastern New Mexico has yet been made with the Entrada of tion is still open to question, some vertebrate (dinosaur) re- the Four Corners area. The present correlation is established mains have been found in a few scattered localities supporting solely on the basis of similarity of stratigraphic sequences in a general Jurassic age assignment. Some palynological evidence the two regions. Until a more definitive correlation is estab- from the upper part of the rock sequence suggests a very late lished, the term "Exeter" should be retained for this rock unit Jurassic or even an Early Cretaceous age (L. R. Wilson, Univer- in northeastern New Mexico. sity of Oklahoma, personal communication, 1972). The Exeter Sandstone crops out in the valley of the Dry EXETER SANDSTONE Cimarron, along the Canadian Escarpment, along the north- facing escarpment of Llano Estacado south of the Canadian The lower part of the Jurassic rock sequence is charac- River, and in buttes, mesas, and other isolated exposures in terized by white to pale-reddish-brown, massive to randomly this region. The distribution of these exposures suggests that crossbedded, friable, fine-grained, well-sorted sandstone (fig. the sandstone was an essentially continuous rock unit prior to 2). The units lower contact is disconformable with the under- its partial removal in the present erosion cycle. lying rocks of the Triassic System (primarily the Dockum A typical stratigraphic section of the Exeter Sandstone is Group), and the upper boundary appears to be conformable characterized by a lower portion with large-scale crossbedding. with the overlying mudstone and shale of the Morrison Forma- This crossbedded portion commonly constitutes approxi- tion. Evidence for intertonguing of Exeter and Morrison lithol- mately the lower two-thirds of the section. This is in turn ogies exists at several places. overlain in apparent conformity by thick-bedded sandstone The name "Exeter" was first applied by Lee (1902, p. 45) layers that display crossbedding in some places, but on a small to a sandstone exposed in the Dry Cimarron Valley. The scale. The exact boundary between these apparently different specific type locality is near the now-abandoned Exter Post sedimentation units is difficult to define at all localities, but Office. Lee erred in his use of the original name "Exter," and where the boundary is discernible the thickness ratio of the Stanton (1905) and DeFord (1927) later attempted to correct two units is not a constant. 91 CANADIAN RIVER GALLEGOS RANCH MITCHELL RANCH SE sec. 35, T.20N ,R 24E. NWsec. 10,T.16N.,R.30 E. NW i sec 16 T 20N. R 29E U) 0U) 0U) LLJ s <1 iHO H 0 cc w cr -200 -150 H w -100 L -50 LL1 cr z w 0 -0 w0 _If x z Vertical scale W < DOCKUM GROUP Mitchell Ranch Canadian River -0 I Gallegos Ranch NEW MEXICO crz EXPLANATION c z ujo - i—cn xxxxxxx "Agate bed" LLI L) I-J- X al..■1 Bentonite UJU) wz Xn Limestone WZ U) base not (f) exposed DOCKUM GROUP Figure 1. Selected strutigraphic sections of Jurassic rocks in northeastern New Mexico. 92 Figure 2. View of Exeter Sandstone butte, looking north, on east side of Ute Creek west of dry-ice plant at Bueyeros (center sec. 28, T. 21 N., R. 30 E., Harding County). Contact with underlying Dockum Group of Triassic age at base of massive sandstone. Exeter at this locality is about 50 feet thick. The boundary of the upper sedimentation unit must be The Exeter may be typically characterized as a sparsely arbitrarily defined, because the sandstone layers are inter- cemented, very well-sorted, fine-grained sandstone. In an bedded with silty shale units that have lithic affinities with the earlier study by the writer (Mankin, 1958) 18 samples were overlying Morrison Formation. This lithologic transition is statistically analyzed for vertical and lateral variations. Sixteen characteristic of most localities and is particularly prominent of the samples were classified as fine sand and the remaining along the Canadian Escarpment (see Road Log p. 27). two as coarse silt.

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