GEOLOGY AND PALEONTOLOGY OF CANAL ZONE Section of strata in middle part of Gatun formation as exposed in Gatun Third Locks excavation [After Thompson and Jones] 12. Sandstone and siltstone d. Massive very fine-grained silty sandstone; thin lenses of conglomerate (thickness a few cm) made up of pebbles of volcanic rocks. Contains leached shells and few carbonized plant remains_-------------------------c. M arly siltstone _____- -- -__----- _b. Clayey marly siltstone. Contains abundant well-preserved shells and some plant remains. Locality 155c _------a. Lens of medium- to very fine-grained silty and marly sandstone_- ..-- _--- _-----11. Massive medium- to very fine-grained, silty and marly sandstone. Contains abundant shells and fragments of carbonized wood. Shells arranged in layers and concentrated in pockets. Locality 155 represents units 11 a n d 1 2 _- -_ _ - _- _- _- ._- _10. Poorly sorted conglomerate, increasingly calcareous upward. Consists of pebbles of dense volcanic rocks (maximum length 10 cm) in matrix of medium-grained sandstone. Contains shells and bits of carbonized wood. Locality 154 -------- -- _-_-_---- __-_-9. Coarse-grained tuffaceous sandstone, consisting principally of grains of dark-colored volcanic rocks, quartz, and pumice. Uppermost 60 cm conglomeratic; thin lenses of conglomerate throughout. Contains a few shells, m ostly leached __- _- _- _- _..-_- _..---____-8. Sandy and silty tuff. Grain size decreasing downward and pumice more abundant downward. Contains a few leached shells7. Hard, brittle, massive, very fine-grained tuff, consisting chiefly of minute glass shards. Contains rounded pieces of pumice (maximum diameter generally 5 cm)___..--___-6. Coarse-grained sandstone, upper part conglomeratic __-----_-..---_-_--- -- _5. Massive medium- to fine-grained, poorly sorted, somewhat tuffaceous sandstone. Contains scattered basalt pebbles, leached shells, and bits of carbonized wood---_ -___ ---. --._-4. Medium- to very fine-grained tuffaceous sandstone, sandy tuff, and very fine-grained tuff in beds 15 to 90 cm thick. Increasingly fine-grained, tuffaceous, and pumiceoiis dow nw ard _--- _..__-- _- __-- _-- -- _-_3. Massive, medium- to very fine-grained somewhat marly sandstone. Glass shards and pumice fairly common in lowest 3 meters, decreasing upward. Contains scattered shells, echinoid fragments, and fragmentary carbonized and calcified plant remains. L ocality 153a ..-_..-.-----------------2. Hard, brittle, massive, very fine-grained tuff_-_- Meters Section of strata in middle part of Gatun formation as exposed in Gatun Third Locks excavation-Continued [After Thompson and Jones] 1. Massive medium- to very fine-grained, silty to so m le Me ter .1 mewhat marly sandstone. Contains nuerous shells, for most part more or less ached. Locality 153 ------------------- 31. 1 Thickness of section------------------- 151-156.5 7.9 3. 3 Units 1 to 4, inlusive, of the preceding section are shown in plate 10. The conglomerate forming unit 10 probably is the same bed as the conglomerate near 8. 4 Gatun described by Howe (1907, pp. 113-114; 1908, 1 5 pp. 228-229). In his 1908 account Howe was tempted to select the conglomerate as the base of the Gatun formation (his Monkey Hill formation of that account), but his view was influenced by faulty paleontologic information. Unit 10 is stratigraphically not far from the base of the Gatun at Gatun Dam spillway as selected by Olsson (1942, p. 244-245). At all events his unconformity between the Gatun and Caimito for- mations, as he now realizes (personal communication), is a minor discontinuity in the middle part of the Gatun formation. 6 Sandy and marly siltstone seem to be the principal constituents of the upper part of the formation, at least in the Mindi (localities 171-173) and Mount Hope (localities 174-178) areas. 7. 2 Both upper and middle parts are represented west of the canal. Farther south-that is, west of Gatun Lake-the upper part evidently is overlapped by the 9. 1 Chagres sandstone and its Toro limestone member. Collections of fossils west of the canal and west of Gatun Lake were made by A. A. Olsson during explora3.6-6 tions for the Sinclair Central American Oil Corporation in 1918, but his map and report are no longer 1. 2 available. Some of the localities at which fossils were collected cannot be plotted on plate 1 and those that are plotted are located only approximately. Olsson's 11. Anomia zone west of Gatun Lake is considered part of the Toro limestone member of the Chagres sandstone, not part of the Gatun formation (Olsson, 1942, p. 246-247). 3. No information is available concerning the Gatun formation between locality 170, west of Gatun Lake near Escobal, and the western end of the outcrop area, where the formation emerges on the coast, as shown in figure 3. Collections of fossils made by geologists of the Sinclair Central American Oil Corporation in 21. 3 1918 indicate that only the upper part of the Gatun 1. 8 is represented in the far western coastal area. That 44 r 3 6