GEO

 i clay-like matrix. The fauna, which includes pelagic Foraminifera, a few pteropods, and a rich assortment of benthonic foraminifera and mollusks, indicates a moderate-depth environment (50 to 100 fathoms; outer neritic zone of forthcoming "Treatise on paleoecology" to be published by the Geological Society of America). The late Miocene upper part of the Gatun in the western area, west of the Canal Zone (fig. 3), is overlapped by the Cliares sandstone, but is presumed to be present in the subsurface section.
 Deposits of early Miocene age are unknown in the ,Gatun Lake district. That they were deposited there is indicated by the Caribbean faunal affinities of the early Miocene part of the Caimito formation in Madden basin. As it is unlikely that they were removed before deposition of the Gatun formation, it is concluded that they are overlapped by the Gatun, which, along the northeast border of the central Panamni area, overlaps the Caimito formation and rests on the Cretaceous(?) basement.
 Except on Barro Colorado Island, the Caimito foriiation consists of moderately coarse, shallow-water, highly tuffaceous rocks and thin algal-foraminiferal limestone. Though the lower part of the Caiito on Barro Colorado includes thun algal-foraminiferal limestone, it is made up chiefly of medium- to very finegrained, somewhat tuffaceous sandstone. These finegrained rocks contain a moderate-depth fauna. At one locality (54n) silty, very fine-grained sandstone contains numerous discoasters and other pelagic coccolithoplhores and numerous pelagic Foraminifera. The outcrop section of the Caimito on Barro Colorado, like that of the Boleo formation, points to progressively deeper water and finer grain size in a seaward direction.
 By the same line of reasoning outlined for deposits of early Miocene age, the Gatuncillo formation is expectable in the subsurface section of the Gatun Lake district. That expectation recently was realized, when the Gatuncillo was indentified at a depth much shallower than expected. In 1955 R. H. Stewart, of the Panama Canal Company's Meteorological and Hydrographic Branch, examined the cores obtained in Core Hole CH-5, drilled in 1946 at a locality in Gatun Lake 325 meters south of Guava Island, a small island of the Brujas Islands group. (The core-hole locality is 1.3 kilometers east of locality 55a of plate 1.) The cores include a considerable thickness of fossiliferous limestone, logged as part the Caimito(?) formation when the core hole was drilled. Mr. Stewart, however, thought it probably is a limestone of the Gatuncillo formation and his suspicion was confirmed when W. S. Cole identified Heterostegina ocalana, Lepidocyclina macdonaldi, L. chaperi, and Asterocyclina georgiana in
 413788-57-5


LOGY


61


a sample of readily disintegrated limestone from the core. A synopsis of the core is as follows:
 Log of Core H1ole CH--5, drilled in Gatun Lake
 Thikess
Lake sediments --_-----_-----_------------5 9
Caimito(?) formation:
 eetotnitic tuff grading downward into sandy siltstone
 is beset 3 in----------------------ii1.2
Gatuncilio formation:
 Hard fossiliferous limestone grading downwvard into
 soft mariy lieestone in lower 1.9 i s ---- 5
 Sandv siltstone and thin beds of tuff and limestone-.. 14. 4


Total thickne,


38. 0


 The presence of the Gatuncillo formation 11.2 meters below the bottom of the lake sediments can hardly be accounted for without in assumption of a major fault trending a little north of east and lving just north of Trinidad Island and just south of the Brujas Islands group. The Caimito formation is not kisonii to overlap the Bohio formation anywhiere in the central part of the Gatun Lake area, although in the northeastern part of the area, north of Nueva Providencia, it evidently overlaps both Bohio and Gatuncillo. The strata in the core hole overlying the Gatuncillo formation do not suggest overlapping deposits. Nevertheless the siltstone strongly suggests the tuffaceous siltstone of the Caimito exposed on the south coast of Pato Horqueto Island (locality 55a, pl. 1), the island in the Brujas group west of the core-hole site. If these suggestions are correct, the core hole passed through a fault at a depth of 29.7 meters below the surface of the lake-a fault having a stratigraphic displacement of about 300 meters. No evidence indicating a fault, however, was recorded by the geologist who prepared the log. As a matter of fact, a fault of that character and of the trend just specified accounts for the marine member of the Boheio(?) formation on Trinidad Island much more satisfactorily than plate 1. The Gatuncillo formation, consisting chiefly of ioderate-depth siltstone and mudstone, is a likely source for oil throughout the central Panama. area and its limestones are suitable reservoirs.
 Madden basin.-The following table shows estimates of outcrop and probable maximum subsurface thickness in Madden basin.
Estimated outcrop and probable maximum subsurface thickness of
 sedimentary rock formations in Madden basin
 Maximum
 Outcrop subsurface
 thickness thickness
 Formation (meters) (7neters)
Caet eite ------------- ---- 459 459
Bebie foratio-------------- 0-290 399
Gatuncilo formation------ -- 300 500
 Total ..---- ..---- .------- 750-950 1. 250