This paper deals with the stratigraphy of the rocks generally known as the “Sillery formation”, chiefly exposed on the south shore of the St. Lawrence below Quebec City. It is shown that paleontologic evidence requires revision of the common belief that these rocks represent a single series of strata of early Canadian age. The fossils indicate that some of the strata are Lower Cambrian, while others are undoubtedly Canadian. A detailed study of significant sections in the Lévis-Chaudière area enables the writer to present a new interpretation of the stratigraphy.
The lower Cambrian appears to be represented by more than 2000 feet of shale and sandstone; a few fossils occur in the upper portion of the series. For this complex the writer proposes the new name, Charny formation, discarding the old name, “Sillery”, whose continued use would cause confusion. A characteristic conglomerate, here named the Ville Guay conglomerate, overlies the Lower Cambrian with-
These strata consist mostly of shale, with some sandstone and conglomerate; they include the graptolite-bearing shales that are so well represented at Lévis. The whole series was styled by Logan as the “Lévis formation”.
On the north shore of the St. Lawrence, Logan observed a series of sandstones and red shales, exposed between Sillery and Cap Rouge and whose thickness he estimated at 2000 feet. These strata he designated the “Sillery formation”. He, moreover, assumed that these rocks should be placed above the Lévis, as exposed on the Island of Orléans, thus giving a total thickness of 7000 feet for this composite section of the “Quebec group”.
Later work by Logan and his collaborators consists mainly in the rather tentative assignment of rocks outcropping in other areas to the different members of the “Quebec group” as defined in the type section, and need not concern us here. However, it must be mentioned that part of the Lévis formation was separated under the name “Lauzon” (Richardson, 1866), and still later the “Lauzon” was added to the “Sillery”. The name Lévis was restricted to some 1200 feet of graptolite-bearing shales outcropping at Lévis, and the “Sillery”, including the former “Lauzon”, extended to all the rest. This is the nomenclature that has generally been used up to the present day.
Richardson (1870) gave a different interpretation of the stratigraphy of the area. The rocks that Logan had included in the “Quebec group” were separated into “Pots-dam” (Cambrian) and “Quebec group” (early Ordovician). Within each of the two series Richardson recognized three divisions which, for the “Quebec group”, were those established by Logan. Richardson's conclusions were founded on insufficient and misinterpreted paleontologic evidence and have not been accepted by later workers; nevertheless, the attempt is noteworthy because Richardson appears to have been the first to realize the heterogeneous nature of the strata known as “Sillery” and “Lauzon”.
The next important step in the study of the problem was accomplished by Ells. In two papers Ells (1889; 1892) gave an excellent review of all previous work, and reached new conclusions that were endorsed by Walcott (1890) and, with minor modifications, have been accepted to this day.
Ells' results showed that so many heterogeneous rocks had been lumped in the “Quebec group” that it now contained strata ranging in age from pre-Cambrian to Upper Ordovician. Hence he proposed that the name be abandoned. As to the original, restricted “Quebec group”, consisting of Lévis and “Sillery”, Ells showed, mainly on the evidence of the graptolites studied by Lapworth, that the order of the strata as given by Logan was inverted—i.e., that the “Sillery” was older than the Lévis. Ells regarded the “Sillery” as Upper Cambrian and the Lévis as Lower Ordovician.
After Ells, Dresser (1912) attempted to subdivide the “Sillery”, at least in part of its outcroparea. Young (1913), Raymond (1913), and Clark (1924) contributed various observations, partly of a paleontologic character, tending to show that there is no sharp limit between “Sillery” and Lévis but that the “Sillery” grades upward into the Levis. These conclusions have spread the almost generally accepted belief that the whole of the “Sillery” and Lévis is Lower Ordovician.
Recently discovered facts show that the above conclusion is unwarranted. Ulrich and Cooper (1938), in their revision of the Ozarkian and Canadian Brachiopoda, restudied that often-quoted fossil, the “Obolella” pretiosa of Billings, the only significant species ever found in the “Sillery” at the type locality. They concluded that this form definitely belongs to the genus Bots/ordia, which is typically Lower Cambrian. More recently, the writer (Rasetti, 1945a) found the first trilobites in place in rocks hitherto assigned to the “Sillery”, a few miles east of Lévis. The fauna includes Bonnia and is unquestionably Lower Cambrian. This paleontologic evidence leaves no doubt that portions of what has been called “Sillery” are Lower Cambrian. It is equally certain, on the evidence of graptolites, that other portions of the “Sillery” are of early Ordovician age.
The purpose of this work is to divide the “Sillery” into its Cambrian and Ordovician portions and to establish the corresponding formations.