GEOLOGY


along the fault. Drilling through the agglomerate, however, shows that some of the dikes are cup-shaped rather than ring-shaped (Thompson, 1952).

 STRUCTURE
 STRUCTURAL HISTORY
 Late Cretaceous or early Tertiary deforiation.--The
strongest deformation in the known geologic history of the Canal Zone and adjoining parts of PanamS took place after eruption of the Cretaceous(?) lavas and deposition of the interbedded sedimentary rocks and before the laying down of the basal part of the Gatuncillo formation in middle Eocene time. At the present time this deformation is poorly dated and the structural pattern it produced will not be known until the basement rocks are studied and mapped. Almost vertical altered tuffs on the Transisthmian Highway between Rio Gatun and Rio Agua Sucia strike northward. The basement rocks elsewhere in the eastern part of the Canal Zone and nearby show a north-south grain (Jones, 1950, pl. 2). The intrusion of the dioritic and dacitic rocks that metamorphosed the lavas and tuffs probably accompanied the deformation.
 Movements during late Eocene(?) to early Miocene
time.-Minor and local movements during middle Tertiary time are indicated by coarse detritus and overlaps. The earliest of these movements, late Eocene or early Oligocene, is suggested by the coarse detritus of the Bohio formation and the overlap of the Bohio on the basement complex in the Pacific coastal area east of the Canal Zone. The source of the coarse detritus may have been beyond the limits of the region covered by plate 1, both to the southwest and northeast.
 Minor movement during the middle or late Oligocene is indicated by overlap of the Caimito formation on the Gatuncillo formation in the northern part of Madden basin and by the presumed overlap of the Caimito on the basement northeast of Gatun Lake. Overlap of the Culebra formation on the Las Cascadas agglomerate points to late Oligocene movement.
 The overlap of the La Boca marine member of the Panama formation on the Bas Obispo formation suggests comparable minor movement during early Miocene time.
 Miocene or Pliocene deformation.-Regional deformation, the second period of regional deformation now recognized, took place during Miocene or Pliocene time. The present structural features of the central Panama area were then formed. The dating is uncertain not only because basic data are still incomplete, but also because the Gatun formation and the Ciagres sandstone do not have an extensive distribution. The Gatun formation, which is of middle Miocene age in the region covered by plate 1, is not known to overlie early


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Miocene deposits at outcrop localities anywhere in the central Panam area. The structural relations between the Gatun formation and the Caimito forcmation-the next older formation in the Gatun Lake and Caribbean coastal areas-are at present uncknown. The overlap of the Gatun on the basement, however, indicates at least minor movement presumably at the end of early Miocene time and the regional deformation probably took place at that time. The Gatun formation and also the early Pliocene Ciagres sandstone dip gently seaward in the relatively narrow coastal strip where those formations are preserved. The distribution of the Caimito formation and the marine member of the Bohio(?) formation in the western part of the Gatun Lake area indicates a pronounced unconformity between the Gatun and those older formations or that the Gatun is separated from them by a fault. A fault is suggested on plate 1. Minor movement, evidently of late Miocene age, is indicated by the partial overlap of the Chagres sandstone on the Gatun formation. The regional deformation may have taken place in middle or late Pliocene time after deposition of the Chagres sandstorle, but that appears to be unlikely.
 How far southward the Chagres sandstone and Gatun formation extended is not known. The Ciagres sandstolce, however, apparently did not extend far. The Toro limestone member, at the base of the Chagres in the northeastern part of its outcrop area, and the Anomia-bearing strata at the base farther southwest are shallow-water deposits. The conglomerate and other deposits at the base of the Gatuns formation between Sabanitas and Maria Chiquita also represent a shallow-water faces. That area, however, is at the east end of the basin. To the southwest, in the Gatun Lake area, the Gatun may have extended considerably beyond its present inland border. The base of the formation east of Zorra Island, however, has not yet been examined and farther southwest the inland border of the formation is under Gatun Lake, and was concealed by extensive swamps before the flooding of the lake. In the area where the inland border of the Gatun is concealed, the middle part of the formation may overlap the lower part, just as farther west, in the region covered by figure 3, the upper part seems to overlap both lower and middle parts. If the formation extended far beyond its present inland border, the submerged area probably passed through Madden basin. Fossils characteristic of the Gatun formation are supposed to have been dredged in Panami Bay off La Boca during the construction of the canal (Li, 1930). Pilsbry, (1931, p. 427-428), who examined the types and figured specimens described by Li, found that the few Miocene fossils, among the Recent species actually dredged i Panama' Bay, are indeed characteristic of the Gatun.