NO. 6 OYSTERS OF THE LOPHA LUGUBRIS GROUP KAUFFMAN 5 



and Austin Chalk (fig. 1). A disconformity of unknown magnitude 

 separates the Eagle Ford and the calcarenite at many places. Speci- 

 mens of L. luguhris from this calcarenite are complete and do not 

 exhibit signs of wear. They appear to have lived during deposition 

 of the calcarenite rather than having been reworked from typical upper 

 Eagle Ford Shale, as some workers have suggested. A similar situation 

 exists in the Juana Lopez Member (Carlile Shale) of the Western 

 Interior. The precise age of the "reworked Eagle Ford" calcarenite 

 has not yet been established, but it appears to be lithologically and 

 faunally the southern equivalent of the Juana Lopez. 



Stratigraphic distribution in the Western Interior. — The strati- 

 graphic distribution of the LopJia lugubris group in the Western In- 

 terior is the same as that observed in the Texas Cretaceous sequence 

 (fig, 1), as are the general progression of ammonites and lithologies. 

 The occurrence of lugubris-like oysters in Colorado, particularly in 

 Huerfano Park, is of great importance, since it is only here that all 

 members of the group have been found together geographically and 

 their stratigraphic relationships accurately established. In this area, 

 Lopha bellaplicata novamexicana occurs sporadically in septarian lime- 

 stone concretions of the upper Blue Hill Shale Member (late Middle 

 Turonian, lower part of the range of Collignoniceras hyatti: fig. 1). 

 L. bellaplicata bellaplicata and the variety A are common throughout 

 the Codell Sandstone Member ("Pugnellus Sandstone" of Stanton, 

 1893 [1894] : late Middle Turonian, upper part of the range of C. 

 hyatti). L. lugubris questionably occurs in the uppermost Codell 

 Sandstone (based on two poorly preserved specimens), and is common 

 throughout the Juana Lopez Member (Juana Lopez Sandstone of 

 Rankin, 1944; a calcarenite or limestone in Colorado) of the 

 Carlile Shale. The upper and lower contacts of the Juana Lopez are 

 disconformable throughout much of its geographic range in Colorado 

 and northern New Mexico. 



In northeastern and north-central New Mexico, and parts of the 

 San Juan Basin, the Coloradoan sequence and distribution of the 

 ostreid elements is similar to that in southern Colorado (fig, 1), The 

 Juana Lopez Member is thicker in New Mexico, however, and locally 

 consists of calcareous sandstone, or of interbedded calcarenites, shales, 

 and sandy units. Lopha lugubris occurs throughout this member. 

 Those from the base are possibly distinguishable from those occurring 

 at the top of the sequence on the basis of relatively longer, coarser and 

 less numerous radiating plicae on the older forms. Not enough 

 specimens are available, however, to prove this statistically. 



