GEOLOGY AND PALEONTOLOGY OF CANAL ZONE AND ADJOINING PARTS OF PANAMA


GEOLOGY, AND DESCRIPTION OF TERTIARY MOLLUSKS (GASTROPODS: TROCHIDAE TO TURRITELLIDAE) By W. P. WOODRING ABSTRACT


 Most of the area covered by the present report lies in the central Panamd area of Tertiary marine sedimentary rocks, which extends obliquely across the trend of the isthmus. The central Panamd area contains a sequence of Tertiary deposits, for the most part marine, ranging in age from middle Eocene to early Pliocene. In the southwestern part of the Canal Zone and farther west a thick sequence of volcanic rocks borders the marine area. In the Gaillard Cut area, along the Panama Canal, the marine and volcanic rocks interfinger.
 The oldest rocks, forming the basement on which the Tertiary formations rest, are more or less altered basaltic and andesitic lavas. Altered tuffs containing microscopic marine fossils are interbedded with the lavas. These basement rocks are of Cretaceous(?) age. Sometime during Late Cretaceous, Paleocene, or early Eocene time they were strongly deformed and perhaps at about the same time were intruded by dioritic and dacitic rocks. This is the strongest regional deformation in the known geologic history of this part of Panamd.
 The middle and upper Eocene Gatuncillo formation rests with marked unconformity on the basement rocks. The Gatuncillo is widely transgressive and is essentially uniform lithologically, consisting principally of fine-grained detrital rocks. There is no indication of nearby volcanism during middle and late Eocene time.
 Volcanism reached a climax during Oligocene and early Miocene time. The volcanic centers, which presumably are now concealed by later volcanic rocks, evidently were located in southern Panamd not far west of the Canal Zone. A tongue of Oligocene(?) volcanic rocks, interpreted to have accumulated at the periphery of a volcanic pile, extends eastward across the canal in the northern part of the Gaillard Cut area. These volcanic rocks, consisting of agglomerate, tuff, and thin andesitic flows and flow breccias, constitute the Bas Obispo formation and Las Cascadas agglomerate. They are considered of Oligocene(?) age because of their inferred relation to Oligocene deposits in the adjoining marine area.
 The Oligocene deposits in the marine area are heterogenous and contain much volcanic debris. The earliest of these deposits are basaltic boulder conglomerate and basaltic graywacke forming the Bohio formation. This coarse debris, directly overlying the fine-grained rocks of the Gatuncillo formation, indicates movements in the source areas. As a result of these movements the Bohio formation overlaps the Gatuncillo formation in the Pacific coastal area east of the Canal Zone. The Bas Obispo formation is thought to grade northward into the Bohio. Though the Bohio represents for the most part an extension of nonmarine deposits into the marine area, it includes marine deposits. Marine upper Eocene or lower Oligocene strata in the western part of the Gatun Lake area are interpreted as a marine member in the lower part of the Bohio(?); the basal part of the Bohio in the Quebrancha syncline includes lower Oligocene marine siltstone; and the upper part of the formation on Barro Colorado Island and in the Pacific coastal area contains thin upper Oligocene marine deposits.


 Late Oligocene time also witnessed the deposition in the marine area of heterogenous strata overlying the Bohio formation. These strata are almost entirely marine and are grouped as the Caimito formation. The Caimito overlaps the Bohio in the northern part of Madden basin and apparently also northeast of Gatun Lake, where it evidently rests directly.on the basement. The overlap indicates continuation of the minor movements that affected the distribution of the Bohio formation. The Caimito formation is made up chiefly of tuffaceous sandstone, tuffaceous siltstone, conglomerate, tuff, agglomerate, and limestone. In the Quebrancha syncline it includes the economically important Quebrancha limestone member, which is quarried for the manufacture of cement. The lower member in the Gatun Lake area (or perhaps the entire formation) is thought to grade southward into the Las Cascadas agglomerate of the Gaillard Cut area.
 Alternating marine and volcanic deposits were laid down in the Gaillard Cut area in early Miocene time. These deposits make up, in ascending order, the Culebra formation, including the Emperador limestone member, the Cucaracha formation, and the Panamd formation, including the La Boca marine member and the Pedro Miguel agglomerate member. Though the Culebra formation contains much tuffaceous material, it consists of dark thin-bedded shale, mudstone, and siltstone; calcareous sandstone, and limestone-all laid down during a minor marine transgression. The Cucaracha formation consists almost entirely of nonmarine tuff, altered to bentonitic clay. Tuff and relatively fine grained agglomerate are the chief constituents of the Panamd formation proper; silty mudstone, sandstone, limestone, and tuff make up the La Boca marine member; coarse-grained agglomerate the Pedro Miguel agglomerate member. The La Boca marine member represents a reinvasion of the sea that transgressed across the Cucaracha and Culebra formations onto the Bas Obispo formation. Two of these lower Miocene formations, the Culebra and Cucaracha, are readily eroded. They form topographic basins between hills of agglomerate and basalt, and these topographic basins determined the course of the canal. The Panamd formation is the youngest Tertiary formation in the Gaillard Cut area and in the Pacific coastal area east of the Canal Zone.
 Tuffaceous sandstone and limestone deposited in Madden basin during early Miocene time are grouped with the underlying strata of late Oligocene age in that area as the Caimito formation. Though the lower Miocene part of the Caimito formation of Madden basin is thought to include the equivalent of the lower Miocene formations of the Gaillard Cut area, there is no satisfactory faunal or lithologic correlation from one area to the other. The lower Miocene formations of the Gaillard Cut area and the deposits in Madden basin considered to be their equivalent represent the early half of the early Miocene, which corresponds to the late Oligocene of some paleontologists. The youngest deposits in Madden basin (the Alhajuela sandstone member of the Caimito formation), however, are younger than the disputed Oligocene or Miocene. Madden basin is the only area where
 1