ABSTRACT

Three partial sections of the lower Ojinaga Formation were measured in the southernmost Quitman Mountains, near the Rio Grande. The lower Ojinaga is composed largely of medium gray to black, variably calcitic shale, except for dark, flaggy calcarenulitic limestone and shale in the basal 20 m and for a few limestones and a dolomite above. Thirty-two residues and 75 thin sections were studied.

The foraminiferal populations are overwhelmingly planktonic; plankton-benthos ratios exceed 99/1 without exception. The fauna consists primarily of Hedbergella amabilis Loeblich and Tappan, H. brittonensis Loeblich and Tappan, H. delrioensis (Carsey), H. planispira (Tappan), H. simplex (Morrow), Rotalipora cushmani (Morrow), R. montsalvensis Mornod, R. brotzeni (Sigal), and R. greenhornensis (Morrow), although Heterohelix occur sporadically and poor preservation obscures possible Praeglobotruncana and Whiteinella. In addition to foraminifers, calcispheres and radiolarians equal or exceed the foraminifers in many samples. In varying proportions the three taxa form minute laminae, often microscopically crossbedded and disconformable in the calcarenulitic biogenic limestones. Juvenile ammonites and protoconchs are common at the top of the section. Inoceramid prisms, oyster fragments, and fish debris occur in the coarser and thicker laminae.

The sequence sampled is generally correlative with the Middle and Upper Cenomanian and the Lower Turonian. A more precise correlation suggests Middle Cenomanian and Lower Turonian with an intervening unconformity, although the taxonomic foundation is questionable.

The microfauna is markedly pelagic and typical of mid-bathyal or deeper depositional environments. The dark color and general lack of bioturbation suggest the possibility of bottom anoxia and consequent absence of benthonic forms, but tests for organic carbon show only a modest elevation of total organic content with a maximum of 2.7 percent.

The microscopic sedimentary structures of the biogenic flaggy limestones testify to gentle but definitely tractional current action at the bathyal depths.