The slates and shales underlie known Carboniferous strata on their eastern border and appear to be a physical continuation of the fossiliferous slates of Bolivia; hence they are provisionally referred to the Silurian, though they may possibly be Devonian. Certainly the known Devonian exceeds in extent the known Silurian in the Central Andes but its lithological character is generally quite unlike the character of the slates here referred to the Silurian. The schists are of great but unknown age. They are unconformably overlain by known Carboniferous at Puquiura in the Vilcapampa Valley ([Fig. 158]), and near Chuquibambilla on the opposite side of the Cordillera Vilcapampa. The deeply weathered fissile mica schists east of Pasaje (see [Appendix C] for all locations) are also unconformably overlain by conglomerate and sandstone of Carboniferous age. While the schists vary considerably in lithological appearance and also in structure, they are everywhere the lowest rocks in the series and may with confidence be referred to the early Palaeozoic, while some of them may date from the Proteriozoic.
Fig. 158—Geologic sketch map of the lower Urubamba Valley. A single traverse was made along the valley, hence the boundaries are not accurate in detail. They were sketched in along a few lateral traverses and also inferred from the topography. The country rock is schist and the granite intruded in it is an arm of the main granite mass that constitutes the axis of the Cordillera Vilcapampa. The structure and to some degree the extent of the sandstone on the left are represented in Figs. [141] and [142].
The Silurian beds are composed of shale, sandstone, shaly sandstone, limestone, and slate with some slaty schist, among which the shales are predominent and the limestones least important. Near their contact with the granite the slate series is composed of alternating beds of sandstone and shale arranged in beds from one to three feet thick. At Santa Ana they become more fissile and slaty in character and in several places are quarried and used for roofing. At Rosalina they consist of almost uniform beds of shale so soft and so minutely and thoroughly jointed as to weather easily. Under prolonged erosion they have, therefore, given rise to a well-rounded and soft-featured landscape. Farther down the Urubamba Valley they again take on the character of alternating beds of sandstone and shale from a few feet to fifteen and more feet thick. In places the metamorphism of the series has been carried further—the shales have become slates and the sandstones have been altered to extremely resistant quartzites. The result is again clearly shown in the topography of the valley wall which becomes bold, inclosing the river in narrow “pongos” or canyons filled with huge bowlders and dangerous rapids. The hills become mountains, ledges appear, and even the heavy forest cover fails to smooth out the natural ruggedness of the landscape.
It is only upon their eastern border that the Silurian series includes calcareous beds, and all of these lie within a few thousand yards of the contact with the Carboniferous limestones and shales. At first they are thin paper-like layers; nearer the top they are a few inches wide and finally attain a thickness of ten or twelve feet. The available limestone outcrops were rigorously examined for fossils but none were found, although they are lavishly distributed throughout the younger Carboniferous beds just above them. It is also remarkable that though the Silurian age of these beds is reasonably inferred they are not separated from the Carboniferous by an unconformity, at least we could find none in this locality. The later beds disconformably overlie the earlier beds, although the sharp differences in lithology and fossils make it easy to locate the line of separation. The limestone beds of the Silurian series are extremely compact and unfossiliferous. At least in this region those of Carboniferous age are friable and the fossils varied and abundant. The Silurian beds are everywhere strongly inclined and throughout the eastern half or third of their outcrop in the Urubamba Valley they are nearly vertical.
In view of the enormous thickness of the repeated layers of shale and sandstone this series is of great interest. Added importance attaches to their occurrence in a long belt from the eastern edge of the Bolivian highlands northward through Peru and possibly farther. From the fact that their disturbance has been on broad lines over wide areas with extreme metamorphism, they are to be separated from the older mica-schists and the crumpled chlorite schists of Puquiura and Pasaje. Further reasons for this distinction lie in their lithologic difference and, to a more important degree, in the strong unconformity between the Carboniferous and the schists in contrast to the disconformable relations shown between the Carboniferous and Silurian fifty miles away at Pongo de Mainique. The mashing and crumpling that the schists have experienced at Puquiura is so intense, that were they a part of the Silurian series the latter should exhibit at least a slight unconformity in relation to the Carboniferous limestones deposited upon them.
If our interpretation of the relation of the schists to the slates and shales be correct, we should have a mountain-making period introduced in pre-Silurian time, affecting the accumulated sediments and bringing about their metamorphism and crumpling on a large scale. From the mountains and uplands thus created on the schists, sediments were washed into adjacent waters and accumulated as even-bedded and extensive sheets of sands and muds (the present slates, shales, quartzites, etc.). Nowhere do the sediments of the slate series show a conglomeratic phase; they are remarkably well-sorted and consist of material disposed with great regularity. Though they are coarsest at the bottom the lower beds do not show cross-bedding, ripple marking, or other signs of shallow-water conditions. Toward the upper part of the series these features, especially the ripple-marking, make their appearance. During the deposition of the last third of the series, and again just before the deposition of the limestone, the beds took on a predominantly arenaceous character associated with ripple marks and cross-bedding characteristic of shallow-water deposits.