Late Capitanian foraminifers are described from two cores taken by Amoco on the north side of the entrance of Dark Canyon. The Amoco #1 core (400ft) penetrates proximal forereef debris of the Tansill Formation and bottoms in massive Capitan reefal limestone. The Amoco #2 core (468ft) penetrates proximal backreef lagoonal deposits of the upper Yates Formation and the Tansill Formation.
Foraminifers are very diverse and abundant in these cores. Based on the fusulinid distribution, the Codonofusiella extensa (upper part of the Yates Formation), Yabeina texana, Paradoxiella pratti and Reichelina lamarensis zones (lower and middle Tansill Formation) are present in the Amoco #2 core. Among small foraminifers, the first appearance of the genera Sengoerina and Crescentia are recorded at the base of the Y. texana Zone and they disappear at the top of the P. pratti Zone. The genus Baisalina appears at the base of the R. lamarensis Zone and disappears at the top of that zone. The Ocotillo Silt Member and upper dolomitized part of the Tansill Formation do not contain foraminifers. The Ocotillo Silt Member is present in the Amoco #2 core at a depth of from 88 to 112ft and, because of its regional dip of approximately six degrees, would possibly project below the base of the Amoco #1 core, thus correlating the Capitan and Tansill formations in the Amoco #1 core with the upper Tansill Formation of the Amoco #2 core. The lower part of the Amoco #1 core (from 110 to 370ft) contains fusulinacean species of the Paraboultonia splendens Zone. The assemblage of small foraminifers is dominated by species of nodosariids and hemigordiopsids. Four new genera and sixteen new species of foraminifers are described from both cores.