GEOLOGY


aneters and in composition from tuffaceons and pebbly alcareous sandstone to sandy limestone. The calareous and sandy strata are separated by dark calcareous or somewhat carbonaceous shale and mudstone. Carbonaceous strata in both parts of the formation contain land plant debris, including identifiable leaves (Berry, 1918).
 Generally the Culebra formation is overlain directly by conglomerate at the base of the Cucaracha formation, marking a discontinuity, evidently a minor discontinuity. On both sides of Gaillard Cut, however, just northeast of the site of Culebra, somewhat calcareous silty sandstone and sandy siltstone interbedded with clay like that of the Cucaracha form a transition .zone between the two formations. The transition zone is included in the Culebra formation. Sluicing operations carried on in 1947 on the west side of the canal, in the region where the transition zone is represented, exposed the section below. Unit 1 is at the level of the canal.



Section of upper part of Culebra formation, including transition
 zone between Culebra and Cucaracha formations, on west side
 of Gaillard Cut at canal station 1759 2 near site of Culebra

Trnsition zone between Culebra and Cucaracha formations: MeS,
 15. Clay, dark-gray, slickensided, and silty carbonaceous clay. Overlain by light-gray mediuigrained locally conglomeratic sandstone taken
 as base of Cucaracha formation __-----__- 4. 6
 14. Siltstone, limonitic-weathering, dark-gray; few
 gypsiferous shell tips of Turritella weathered
 out __--_--___-----___- __-_----------- .
 13. Siltstone, dark-gray, sandy; includes a 15-cie
 fossiliferous somewhat calcareous layer at base (locality 112) and fossiliferous calcareous concretions at and within 30 cmi of top
 (locality 112a) -_--_ __----_--------- 1. 0
 12. Clay, greenish-brown, slickensided, silty ---___ 5
 11. Sandstone, greenish-gray, silty, medium-grained;
 and siltstone ------------_--------- - 1. 6
 10. Sandstone, broNitish-gray, silty, mediumgrained; and siltstone containing petrified
 wood _-_-_-_--------------- 1. 1
 9. Clay, limnioniitic-weathering, slickensided, darkgray ----_-_--------- 1.3
 8. Clay, grayish-green, somewhat carbonaceous
 and somewhat fissile ------ 2. 3 Culebra formation proper, upper part:
 7. Sandstone, light-gray, medium-grained, poorly
 sorted, silty, calcareous; siltstone partings-_- 1. 5
 6. Shale, dark-gray, silty, somewhat carbonaceous;
 includes thin layers of calcareous sandy siltstone -----_---------_ --.------- 1. 3
 3 The canal stations are located at intervals of 100 feet (30 meters) along the center alinement and are numbered from the Caribbean terminus to the Pacific terminus. Strictly speaking the rock exposures are opposite the stations, not at them.


35


Section of upper part of Culbra formation, including transition
 zone between Culebra and Cucaracha formations, on west side of Gaillard Cut at canal station 1759 near site of CulebraContinued Meer


5. Sandstone, light-gray, medium- to coarsegrained, poorly sorted, calcareous; siltstone
 partings
4. Shale, dark to black, calcareous, somewhat
 carbonaceous
3. Sandstone, light-gray, medium- to coarsegraited, poorly sorted, calcareous, it beds 30 to 90 cim thick and interbedded with poorly exposed somewhat carbonaceous shale.
 Includes a 15-ct layer of conglomerate - 2. Sandstone, coarse-grained to conglomeratic,
 calcareous; contains a ioderately large smooth species of oyster -- _-- -_----1. Sandstone, light-gray fine- to tmediu-i-grained,
 poorly sorted, calcareous, in beds 30 cm thick and interbedded with dark to black calcareous and carbonaceous shale and sudstone. Locality 108, 1.5 tn above edge of c a n a l _-- - - .- .- - _- _- - _- .. .


3. 2 1. 4 51









4. 2


 Thickness of section--------- -----33. 5

 Part of the transition zone is exposed on the east side of the canal at canal stations 1754. Fossiliferous strata corresponding to the fossiliferous parts of bed 13 of the preceding section are recognizable on the east side, but the best-preserved fossils are weathered out and were put in one collection (locality 110). Silicified wood is common at that locality, including segments of logs riddled with shipworm tubes (Teredo).
 Limestone of Emperador type in the upper part of the Culebra crops out farther northwest on the west side of the canal on both limbs of a syncline near the site of Las Cascadas. At locality 120 (canal station 1600) the limestone is 6 meters thick and in a nearby core hole is 24.3 meters above the base of the Culebra. At locality 121 (canal station 1619, pl. 8) the thickness is 15.2 meters and the limestone is about 27.5 meters above the base of the Culebra. At both localities the underlying strata consist of dark carbonaceous clay and tuffaceous siltstone. The basal 30 to 60 centimeters of the limestone at locality 120 is silty and contains numerous pectinids. The limestone at locality 121 includes a basal calcareous siltstone bed that has a thickness of 15 to 30 centimeters and a middle calcareous siltstone bed 2.4 meters thick. The limestone at these two localities appears to represent the same zone and probably is the same as limestone near Tower N, a signal tower on the pre-construction line of the Panama Railroad near Las Cascadas. Fossils from "the Pecten bed" near Tower N were recorded by Brown and Pilsbry (1913, p. 502-503). The limestone in the Las Cascadas area is presumed to be the equivalent of