TheMiddle Jurassic Summeruille Formation, northern New Mexico by0. J. Anderson,New Mexico Bureau of Minesand Mineral Resources, Socono, NM 87801; andSpencei G, Luns,New Mexico Museum of NaturalHistory, 1801 Mountain Road NW, Albuquerque, NM87104 Abstract name used in New York state at the time it was introduced in Colorado. Clearly, usage of the name Wanakah outside of New We recommend continued usage of the name Summerville For- York should be discontinued. mation as a stratigraphic unit in the upper San Rafael Group in In northeastem New Mexico, the thinly bedded stratathat overlie northern New Mexico. Our reasonsare as follows. Subsequentto the lower San Rafael Group have been designated as Bell Ranch recognition in 1947of the Entrada Sandstoneand overlying Todilto Formation since 1959. Based on similarity of lithologies, deposi- Formation (the lower San RafaelGroup) in the southern San Juan tional environments, and stratigraphic sequenceand position, it Basin of northwestern New Mexico, field-oriented geologistsgen- is now virtually certain that the Bell Ranch is equivalent to and erally agreed to regard the overlying rocks as the upper part of the westward correlateswith Summerville strata. We thus recommend San Rafael Group. Although fossil control was lacking, the strati- that the name Bell Ranchbe abandonedand the name Summerville graphic sequenceand position were right, and lithologies and fa- retained for upper San Rafael Group strata acrossnorthern New cies were similar. Hence, until 1986, Summerville was used for Mexico. This simplifies the nomenclatureand properly relatesMid- these upper San RafaelGroup strata in all geologic mapping, and dle Jurassicrocks of the High Plains to the Colorado Plateau se- the usage becameentrenched. quence. In 1986, the name Wanakah Formation was introduced for the Other questions regarding correlation and nomenclatureof Mid- strata previously designated and mapped as Summerville in the dle Jurasiic units--Curtis, Pony Expressbeds, Bilk Creek, Bluff, San Juan Basin of New Mexico. The Wanakah type area is in the Cow Springs, Junction Creek, Zuni. and Tidwell-are discussed Ouray dishict of southwestemColorado, where the strataso named as they relate to the Summerville Formation' had been recognizedas equivalent to the Summerville Formation; the usageof Wanakahwas local only. Sinceequivalency with Sum- Introduction merville can be demonstrated, no scientific purpose is served by replacing the more widespread name Summerville with the pro- This report discusses the Summerville Formation and associ- vincial term Wanakah. Of greater significance, however, is that ated strata of the San Rafael Group (Middle jurassic) in northern Summerville has precedence,and that Wanakahwas a stratigraphic New Mexico and adjacent areas (Fig. 1), with Particular respect 108" 106' 104" Moab It o Monlrose ?iver co Nl n,l Beclablto Horse Mesa \t i D/t\ 'lf;"tfitl l Church Hasan ul Rock '\, /i (t $ Basin \\in l J\ Y>,f\l" Zuni . )l aAlbuquerque Pueblof;\ r \\" t\ i# 110' 108" 106' FIGURE1-Index map of the study areashowing outcropsof Middle Jurassicrocks (J, shaded)and someareas of pre-Jurassic(pre-J) and Cretaceous (K) rocks. Nan Merico Geology November 1992 to 1) lithologies, 2) depositionalenvironments, 3) regional cor- and Wanakah in the southwesternpart of the statewith no criteria relations, and 4) the concept of a climatic change marking the given for distinguishing the two. The only early U.S. Geological transition from Middle to Late furassic time. Survey publication to recognizeWanakah was Wood et al. (1953), The Summerville consistsof alternating fine-grained sandstone and curiously this was in northeastern, not northwestern New and gypsiferous siltstone or mudstone that overlies the Curtis Mexico. They were thus the first to recognizethe Wanakah/Sum- Formation and forms the uppermost unit of the San RafaelGroup merville strata on the High Plains. Griggs and Read (1959)pointed in the San RafaelSwell of Utah. To the south, near Moab and La out that since 1953,usage of the name Wanakah had been limited Sal function, the unit thins significantly acrossthe sill or divide to the type area in southwestern Colorado, and they Proceeded that separatedthe marine basin of Curtis deposition to the north- to introduce the name Bell Ranch Formation for strata overlying west from the salina-lakebasin of Todilto deposition to the south- the Todilto in northeastern New Mexico. east. Still further southward near Bluff, Utah, the Summerville More recently Condon and Peterson(1985) and Condon and thickens but retains much of the lithologic characterof the type Huffrnan (1988)"reinstated" the name WanakahFormation, which area, though it is somewhat more sandy. It includes an eolian- they asserted "replaces the name Summerville Formation" sandstone lithosome at Bluff-the bed at Butler Wash-that has throughout northwestern New Mexico. They alleged this change been regarded as a tongue of the Entrada Sandstone.Also at in nomenclafure was necessarybecause of shatigraphic reports Bluff, a prominent sandstoneunit overlies the Summerville, and (O'sullivan, 1980;O'Sullivan and Pierce, 1983)indicating that all is appropriately named Bluff Sandstone.The Bluff Sandstonehas the Summerville Fotmation was cut out by an unconformity in been assigned to both the San RafaelGroup and to the Morrison southeastem Utah. But, in actuality, the Summerville is not cut Formation (Gregory, 1938; Craig, 1955; O'Sullivan, 1980). This out, although when O'Sullivan (1980;1981) realized this he had conflict has arisen because the Bluff contains both eolian and already decided to use the name Wanakah for these strata; this fluvial facies, and becausean unconformity has been perceived is documented by personal correspondence(1986) as well as the at the baseof the Bluff by those who would have it in the Morrison regional cross sections (Fig. 2). In essence,the Summerville is Formation. In this report we treat all eolianites overlying the continuous throughout the area and can be recognized in north- Summerville as essentially contemporaneous dune-field migra- western New Mexico on the basis of lithology, sequence, and tion onto the arid, sandy plain formed during and following the stratigraphic position. Condon and Huffman (1988) also 1) re- northward regressionof the epicontinental Curtis sea.Thus, they duced the Todilto Formation to a member-rank unit at the base all have affinities to and are included in the San Rafael Group. of the newly establishedWanakah Formation, 2) introduced the No major unconformity, and in most instancesno demonstrable name Beclabito Member for the overlying Summerville, and 3) unconformity, exists between the eolianites or mixed eolian-flu- replaced the name Bluff Sandstone with Horse Mesa Member, vial sand bodies and the underlying Summerville. These eolian- which forms the upper part of their Wanakah Formation. They dominated units include strata that have been referred to as Bluff. then establisheda referencesection in New Mexico using member Horse Mesa Member of the Wanakah, Junction Creek Sandstone names not used in the type area, and without benefit of an ac- (Colorado), RecaptureMember of the Morrison, Zuni Sandstone, companying stratigraphic section from the tyPe area. and upper part of Cow Springs Sandstone.Overlying the eolian- ites, or in places typical Summerville, is the fluvially dominated sequence regarded by most as the Morrison Formation. These Statement of problem sediments indicate increasedclastic input during a Late Jurassic The Summerville Formation overlies the Curtis in the type area gradual change to more humid climates. and demonstrably overlies the Todilto in northern New Mexico. Southeastward from Bluff, Arizona and into New Mexico, the In both areas the Summerville has been described as a thinly Summerville is present and continuous acrossthe Four Corners bedded unit consistingof arid coastalplain or sabkha,supratidal platform, the San|uan Basin, and the northeastern portion of the or hypersaline marine, wave-dominated deltaic (McKnight, 1940), state. Throughout much of this areathe Summerville overlies and and lacustrine deposits. These sediments were thus regressive overlaps the Todilto Formation. We discussthe equivalent strata and prograded across facies deposited during a high sealevel in southwestern Colorade-the Wanakah Formation----onlv1) to stand that was synchronousin the Curtis and Todilto depositional first describe in detail a type section, 2) to demonstrate how the basins. In the southern San Juan Basin and adiacentnortheastern Wanakah Formation units, the Pony Expressand the marl beds, Arizona, the Summerville grades southward and southwestward can be correlatedwith the Todilto Formation and the Summerville into an eolian sandpile that has been called the Cow Springs Formation, respectively, and 3) to lend our support to previous Sandstone. workers who have correlated the overlying Junction Creek Sand- Our interpretation of this physical stratigraphic sequenceis that stone westward with the Bluff Sandstone. the onset of regressiveconditions in the Todilto salina basin and the Curtis Seawaywas synchronous and initiated by relative sea- level fall. Ensuing depositional environments in both basinswere Previous sfudies similar, with eolian processesdominating along the southwest An exhaustive review of previous work on furassic rocks of the margin of the integrated basin. It is therefore in the interest of Colorado Plateau area is not undertaken here, as it has recently regional