SUMMARY OF PALEOECOLOGIC INTERPRETATIONS BASED ON FORAMINIFERA
The abundance of planktonic Foraminifera known from parts of the Crescent formation suggests that sometime following the Cretaceous period and prior to late early Eocene or middle Eocene time, open-sea conditions probably prevailed in the area. The absence of benthonic forms in these assemblages suggests that the environment was unfavorable for the existence of bottom-dwelling Foraminifera. The distribution of planktonic Foraminifera is largely dependent upon ocean currents; therefore, their presence in the Crescent formation suggests that they were brought into the area even though conditions were not continuously favorable for their existence. The concentrations of planktonic foraminiferal remains in certain places in the formation suggest periodic annihilation. Conditions for mass destruction of planktonic Foraminifera could have resulted from extensive submarine volcanic activity that is indicated by the numerous pillow basalt flows in the Crescent formation.
In late early to middle Eocene time shallow protected bays existed at least during the deposition of the upper part of the Crescent formation, as is indicated by the presence of benthonic Foraminifera that required shallow, warm, clear water. These conditions may have resulted from the extrusion of volcanic material to the extent that the seas became shallow and in places broken by barrier reefs or islands.
In the early part of late Eocene time the area probably underwent considerable subsidence, because all foraminiferal assemblages known from the overlying Aldwell formation indicate a cool, deep-water environment. This postulation is supported by the almost total absence of megafossils and the fine-grained nature of the sediments of the formation.
The absence of Foraminifera in the overlying Lyre formation may be due to a rapid deposition of the coarse clastic sediments of this formation. Such sediments most probably were deposited under turbulence which, in turn, would have created muddy seas, a condition unfavorable for most Foraminifera.
Following the deposition of the Lyre formation and during late Eocene time, the lower part of the Twin River formation was probably deposited at various depths, but generally in a moderately shallow sea. If the Aldwell-like assemblages, found in a few places in the lower part of the Twin River formation, were not reworked, considerable depths existed locally in the area during that time. However, generally the Foraminifera of the lower part of the Twin River formation suggest moderately shallow depths, upper most bathyal to lower neritic.
By Refugian time (late Eocene or early Oligocene), deepening of the sea continued, although in places, particularly to the west, moderately shallow depths continued to prevail.
Maximum depths of deposition for the Twin River formation were probably attained throughout most of the mapped area during the early part of Zemorrian time (late Oligocene or early Miocene). The Foraminifera suggest that the sea was shallowest in the western part of the area but that it gradually deepened during Zemorrian time. By the close of Zemorrian time the sea rapidly became more shallow.
The much greater thickness and generally coarser grained nature of the Twin River formation in the vicinity of Deep Creek, as compared to the formation in the Lyre River area, suggest that a source of sediments may have been closer to the Deep Creek area than to the Lyre River area. Furthermore, combined information on the paleoecology and age inferred by Foraminifera suggests that shallow conditions existed for a longer period of time in the Deep Creek area than they did in the Lyre River area.
In the Deep Creek section, foraminiferal information is available only from the middle member, a lithologic unit primarily of bedded siltstone and sandstone. Foraminifera indicate that all but the upper 500 feet of this member is early Zemorrian in age. The upper 500 feet is late Zemorrian in age. In the Lyre River section to the east all the Foraminifera of the middle member, except those from the upper 300 feet of the member, are Refugian in age. Foraminifera from the upper 300 feet of the middle member and those from almost all the upper member in the Lyre River section are of early Zemorrian age. The upper member of the formation is chiefly mudstone. Therefore, the siltstone and sandstone of the middle member were deposited during the Refugian time in the Lyre River area, whereas in the Deep Creek area their deposition continued throughout much of Zemorrian time to the west. The sediments of the middle member not only were deposited for a longer period of time in the Deep Creek area, but their greater thickness in a unit of time indicates they also accumulated more rapidly there than they did to the east in the Lyre River area.
These comparisons show that, although the three members of the Twin River formation maintain a stratigraphic sequence, the geologic age as well as the amount of time represented by any one member can vary throughout the mapped area.
Limited foraminiferal evidence indicates that during Saucesian time (early Miocene) at least part of the Clallam formation was deposited under shallow, cool water conditions in a protected environment. Coal beds in the upper part of the Clallam formation indicate that other parts of the formation are continental deposits.
Last Updated: 28-Mar-2006