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Drilling on Leg 77 resulted in eight single-bit holes at five sites extending across the Yucatan Channel in the southeastern Gulf of Mexico. Sites 535, 539 (aborted hole of 7.5 m depth), and 540 were grouped as basin sites, whereas location on up-faulted blocks at Sites 536, 537, and 538 led to their grouping as basement sites. The sedimentary record encountered across the Channel is short, intermittent, and laterally erratic, reflecting both the general instability of sedimentation and the depositional contrast between basin and basement sites. The combined record for the Cenozoic is incomplete, and that for most of the Upper Cretaceous is missing. The Lower Cretaceous record is relatively long at basin sites, including apparent Berriasian at Site 535, but the record is still discontinuous and the biostratigraphic data are sparse, frequently contradictory, and complicated by displacement and reworking. The Lower Cretaceous record is again short and erratic at the basement Sites 536, 537, and 538, where displaced shelf grainstones, with very poor recovery, and variably minor interbedded lime mudstones and marls lie on continental basement. Cenozoic cores are mostly pelagic calcareous oozes, except at Site 535, where more than 150 m of upper Pleistocene distal fan and deltaic mudstones were drilled. The Cenozoic successions are thin, fragmentary, and variable laterally, except for the Oligocene, which is well developed in basin Hole 540 and basement Hole 538A. Planktonic foraminifers are typically abundant and well preserved, but reworking is persistent. No Cenozoic except Pleistocene was encountered at basin Site 535. The Cretaceous/Tertiary boundary was recovered in Hole 536, where a complete foraminiferal zonal sequence of the lowermost Paleocene overlies ill-defined Maestrichtian without perceptible lithic change. The Maestrichtian through Turanian interval is represented by only a few meters at Sites 540, 536, 537, and 538, and ranges from upper Maestrichtian to Santonian. Nannofossil-foraminiferal chalk is typical, but volcanogenic sandstone and coarse bioclastic limestone also occur in relative abundance. No record of this interval was found at Site 535. The Cenomanian is missing at basement Sites 536, 537, and 538, but is thick and gradational into the Albian at basin Sites 535 and 540. The passage into the Albian at Site 540 is accompanied by a change from coarse skeletal to pelagic limestones. The Cenomanian and Albian of Site 535 (535-17.CC to 535-43,CC) form a continuous, mystifying succession that may be a huge slump or canyon fill. The Lower Cretaceous of basin sites differs markedly from that of basement sites. At the latter, the Albian to Aptian displaced shelf grainstones and minor interbeds of lime mudstone are typical. Rare calpionellids in grainstone matrix, displaced trocholinid grains, and rare planktonic specimens in interbeds and grainstone matrix provide approximate ages for layers proximal to the basement, layers which range from upper Aptian (Site 536) to Valanginian (Site 537) and Berriasian (Hole 538A). The Cretaceous of Site 535 is the most complex of the major intervals drilled during Leg 77. The sequence from 535-17,CC, at the base of the Pleistocene, through Section 535-42-2 (240 m) is composed of laminated limestones with some discrete biogenic turbidites and uncommon marly interbeds. Displaced and reworked biogenic materials are common, particularly in the turbidites, but definitely autochthonous planktonic foraminifers are infrequent and rare. Those found indicate a normal succession of middle and lower Albian foraminiferal zones, but rare ammonitic material indicates that the interval is Cenomanian to upper Albian. This contradiction, the extensive reworking, and the rare, small size-graded planktonic foraminifers undermined the validity of the planktonic zones and raised the possibility of an exotic origin of the interval. At Core 535-43 the lithofacies changes to seemingly autochthonous limestone and marls with periodic dark organic and pyritic marls, which continue to total depth at Core 535-79. Small recrystallized planktonic foraminifers are common in indurated limestones at the top (Cores 535-43 to 535-47), indicating an Aptian succession, but planktonic specimens are tiny and rare below. Calpionellids appear in Core 535-64 and become common to abundant in Core 535-71, below which calpionellid Zones E and D occur. Their temporal implications are contradicted, however, by rare Globuligerina and Trocholina, which suggest a lesser age than that implied by the calpionellids, and in turn undermine the ages inferred for the medial part of the interval. In the end, the discontinuous but normal succession of planktonic foraminifer and calpionellid zones identified for the Cretaceous of Site 535 during shipboard and early post-cruise work has become suspect; and the composition of the sedimentary record, the nature of the depositional environment, and the possible structural effects are uncertain. INTRODUCTION the southeastern Gulf of Mexico (Fig. 1). Sites 535, 539, and 540 are on the Florida side; Site 536 is on the CamDuring Leg 77, marine sediment spanning the interpeche side; Sites 535, 539, and 540 were considered baval from Recent to lowermost Cretaceous was recovered sin sites; and Sites 536, 537, and 538 were considered from eight single-bit holes in the Yucatan Channel of basement sites because of their position on high-standing basement blocks. i 7 777 Recent through Tertiary sediments are in general rep1 Buffler, R. X, Schlager, W., et al., Init. Repts. DSDP, 77: Washington (U.S. Govt. , , i A o -. ae u printing office). resented by calcareous oozes , except at Site 535, where

Planktonic Foraminifers and Calpionellids from Gulf of Mexico Sites, Deep Sea Drilling Project Leg 77