The Soleduck formation of Reagan (1909) is stratigraphically the lowest unit known to crop out in the area and consists primarily of argillite and graywacke with minor amounts of conglomerate, arkosic sandstone, and reddish calcareous argillite in association with volcanic rocks. Within the mapped area, the formation crops out only in the area south of a line extending generally east and west of the south shore of Lake Crescent (pl. 1). The thickness of the formation is unknown, but in the mapped area it is estimated to be about 16,000 feet. However, these rocks are believed to overlie similar rocks to the south, and the formation may therefore be much thicker. Although stratigraphically diagnostic fossils have not been found in the formation, its age is tentatively assigned as early Eocene (Brown and others, 1960). No Foraminifera from this formation were studied in connection with the present report.
In some places the Crescent formation conformably overlies, and in other places it interfingers with, Reagan's Soleduck formation. It consists primarily of pillow basalt, flow breccia, amygdaloidal basalt, and waterlaid tuff with minor amounts of tuffaceous sedimentary rocks and reddish calcareous argillite. In the area mapped by Gower (1960) and by Brown and others (1960) the formation is exposed chiefly in a belt that extends from Round Mountain on the east to Deadmans Hill on the west. It is also exposed on the north flank of the Clallam syncline between the mouth of Whiskey Creek and the Elwha River.
The formation varies considerably in thickness and generally becomes thinner to the west; its greatest thickness in the southeastern part of the mapped area is estimated to be 25,000 feet, and the thinnest section, which is in the extreme western part near Bear Creek, is estimated to be 6,500 feet thick. Foraminifera from 11 localities indicate that the Crescent formation is post-Cretaceous in age and may be, in part at least, as young as middle Eocene.
The Aldwell formation conformably overlies and interfingers with the Crescent formation. It consists mainly of well-indurated marine siltstone with lesser amounts of standstone and volcanic rocks. Discontinuous outcrops of this unit are present south of the Clallam syncline and extend from east to west across the area (pl. 1). The Aldwell formation has a maximum thickness of about 3,000 feet in the vicinity of Lake Aldwell, but it is thinner elsewhere and in some places it is overlapped by younger rocks. The age assignment of the formation is based largely on Foraminifera, and they suggest an early late Eocene age.
The Lyre formation as redefined by Brown and others (1956) conformably overlies the Aldwell formation and consists chiefly of conglomerate and sandstone. It crops out in the central part of the mapped area in a belt that extends from about a mile east of the Elwha River to beyond the west border of the mapped area. The thickness varies from more than 3,000 feet in the East Twin River area to only a few feet in some places; in other parts of the area the formation is entirely absent. Although no Foraminifera were obtained from the Lyre formation, assemblages of late Eocene age were collected from the rocks that crop out immediately above and below the formation.
The Twin River formation, as redefined (Brown and Gower, 1958) overlies the Lyre formation and older rocks. Its outcrop belt occupies approximately the northern half of the mapped area and extends beyond both the east and west boundaries. The Twin River formation is divided into a lower, a middle, and an upper member. The lower member is chiefly thin-bedded siltstone and sandstone and has a maximum thickness of 7,500 feet and an average thickness of about 2,000 feet. The middle member is primarily massive to thin-bedded siltstone and contains abundant concretions. In places it is estimated to be as much as 5,000 feet thick, and it has an average thickness of about 2,000 feet. The upper member is predominantly massive, semi-indurated mudstone and sandy siltstone but includes some beds of calcareous sandstone. Its thickness is about 3,500 feet in the western part of the area, where its upper contact with the Clallam formation is exposed. Foraminifera from the Twin River formation range in age from late Eocene to late Oligocene or early Miocene.
The Clallam formation is the youngest known Tertiary unit in the area. It generally crops out only in the area north of Last Creek and west of Pillar Point. The formation is composed primarily of poorly sorted gray fine- to medium-grained thick-bedded sandstone with minor amounts of conglomerate and sandy siltstone. It is estimated to be more than 2,500 feet thick. A Miocene age has been assigned to the Clallam formation, on the bases of both the mollusks and Foraminifera.
The principal structural feature of the area is the Clallam syncline. Its axis has been mapped from a point near the mouth of Murdock Creek eastward to the east boundary of the area. It is a west-trending asymmetric fold with steeper dips along its south limb. Smaller east-plunging folds are also present, particularly in the eastern part of the area. The Soleduck formation of Reagon is intensely deformed but no large continuous folds have been traced. Most of the major faults are parallel to the fold axes and trend generally westward. Some of them show stratigraphic displacements of as much as 5,000 feet. A few north-trending faults are also present, particularly in the eastern part of the area. No major faulting is believed to affect the measured sections; therefore, faults are not shown on plate 1.
Last Updated: 28-Mar-2006