Gatun Lake and environs embraces about 650 square miles, in which the oldest rocks are closely jointed, folded, and partly metamorphosed pre-Tertiary Basement Complex. Over this old rock mass in the lower areas lies a Tertiary sequence of sedimentary, igneous, and pyroclastic rocks ranging from Eocene to late Upper Miocene. Quaternary deposits of unconsolidated sediments ranging from Pleistocene to Recent fill broad drowned areas and principal river valleys, locally to at least 300 feet below sea level. All sedimentary formations are fossiliferous. On a stratagraphic and lithologic basis, the following general correlation is made with other formations outside the lake area.  

An exposure in an excavation at Gatun permitted more detailed stratigraphic and lithologic examination than was possible anywhere else in the area. Here a section of the Gatun formation 546 feet thick consists of marly, tuffaceous, highly fossiliferous sediments laid down in a shallow sea. At least three minor depositional cycles are represented.

Remote areas adjoining the Gatun Lake area contain previously mapped little-known formations. Among these are:

  • (1) The Eocene Tranquilla (Gatuncillo) formation. Discovery of its extension into the Gatun Lake area is the first record of Eocene sedimentary rock in the Canal Zone.

  • (2) The Oligocene-Lower Miocene Quebrancha formation, considered to be stratigraphically equivalent to the Caimito formation.

  • (3) The Lower Miocene Alhajuela member of the Caimito formation, which is the Gatun (?) formation of Reeves and Ross' Madden Dam Report.

  • (4) The Middle Miocene (?) Sabanitas formation, which is probably a continental and shallow-water facies of the lower Gatun formation.

The structure of this area is dominated by fractures, a few of uncertain nature but most believed or known to be faults. The greatest faults have a large normal throw. Numerous small faults apparently associated with Tertiary volcanic eruptions have a large strike-slip. The fracture pattern of the Gatun Lake Area, believed representative of the entire central Panamanian Isthmus, fits the view that the Panama Isthmus Ridge is a wide upwarp between the Caribbean and the Ecuador seas, possibly dating from the orogenic epoch of Senonian time.

The topography of the North Pacific Ocean and adjacent northwestern South America contrasts greatly with that of the Isthmus and the Caribbean Sea beyond, as if the southwestern rim of the Caribbean Basin (southern Isthmian continental slope and the line of its eastward extension) constituted a buttress against Cenozoic compressive force from the southwest and had been fractured and faulted by the force of the compression.

The Gatun Lake area is a physiographic basin flanked by high hills on the east and west and by low ones on the south. The highest hills rising over large areas to more than 1000 feet elevation are formed by pre-Tertiary Basement Complex. The main low part of the area is developed from Tertiary and Quaternary material and may be classified as:

  1. Swamp and lowland areas up to 25 feet A.T.

  2. Interlowland areas up to 300 feet A.T. (500 feet A.T. at extreme points).

  3. Dissected highland up to 700 feet A.T.

  4. Hills region up to 1100 feet A.T.

The history of the area is recorded in four sets of sediments separated by three major unconformities. The first set of sediments, those incorporated in the Basement Complex, is too folded, broken, and crushed to decipher. The second set, those of Upper Eocene through Lower Miocene, is moderately faulted and folded. The third set of sediments is the Middle Miocene and Upper Miocene (?) coastal plain beds which reveal a simple history that could be easily worked out in greater detail. The youngest set is composed of the Pleistocene and Recent unconsolidated sediments which are so young their short history is that of slight uplift.

The general faunal aspect at some of the rare fossil localities proved a welcome basis for distinguishing Upper Eocene, Oligocene, Lower, Middle, and Upper Miocene, and Quaternary deposits.

A system of sedimentary rock terminology suitable for use in geological mapping in this area of pyroclastic, clastic, and chemical sedimentary rocks is described and used throughout the paper.

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