8-13D-9-10: GEOLOGIC OVERVIEW:

Ancestral streams carrying sediments from the adjacent uplands were also deposited near the lake margins.

Coarse-grained sands were deposited along the stream channels and shores of the lake. Finer sands and silts were carried down the Dry Creek drainage system. The sediments that were deposited are collectively known as sediments of the Idaho Group. Because of the depositional complexity of the sediments in the Boise foothills (which includes the Dry Creek area), recent lithostratigraphic formation name changes within the Idaho Group have been reassigned the Terteling Springs Formation. The formation consists of four (4) distinct lithologic members of which portions are found in the study area.

Faulting And Seismicity: Foothills faulting generally occurs in NW/SE and/or east- west trending normal faults. Distinct separation of foothills and valley floor often make fault traces. The exception in the Dry Creek area is separation of lithologic units or nonconformable lateral extension of rock and soil formations. Splinter faults subparallel and often perpendicular to the primary faults have helped define the present geomorphology of the area. Faulting in the foothills is known to consist of single fault plains a few inches wide or fault zones, several feet to tens of feet wide that are composed of many interconnected fault plains. Field observation collected during preliminary site reconnaissance's imply that Dry Creek may be structurally controlled. Observations include linear correlation of primary structures such as contorted bedding plains, hot springs, and unconformable bedding.

Woodward-Lundgren and Associates identified most foothill faults as inactive. According to Woodward-Lundgren, there has not been historical movement of the major faults in the area within the last five hundred thousand (500,000) years. The U.S. Coast and Geodetic Survey in their Seismic Risk Map (1969) and the Uniform Building Code (1982) assign the area to Zone 2B, indicating moderate potential for earthquake damage. Zone 2 corresponds to a maximum earthquake intensity of VII on the modified Mercalli intensity scale. Damage in the study area would probably be caused by ground shaking due to fault displacement outside the study area. Miller and Jones (1988) performed a probabilistic estimation of earthquake intensities for Idaho based on the historic seismic record. They concluded that there is better than a ninety percent (90%) probability that the maximum earthquake intensity will be less than VI for the foothills area for a given fifty (50) year period.

Biotite Granite And Granodiorite:

Areas along the northern and northeastern area of the Hidden Springs property is underlain by intrusive igneous rocks of the Idaho batholith. Biotite granite and granodiorite are the dominant rock types. The granites are typically light gray and medium to coarse-grained. The fracture spacing varies from a few inches to several feet. The granitics vary from slightly weathered to decomposed. Differential weathering of the batholithic rocks have exposed large, rounded outcroppings east of Cartwright Road north of Dry Creek.

Terteling Springs Formation:

The Terteling Springs Formation is a facies assemblage of sediments and sedimentary rock mostly of lacustrine or lakeshore depositional environment. The base of the formation is mapped where these sedimentary deposits rest upon a thick sequence of plagonite tuff and basalt, or upon rhyolite or the granitic rock. The formation is composed of tan to light gray silts, sands, and clays. Induration is moderate, although very dense, silica-cemented sandstone is locally present. The silica cement was precipitated from hydrothermal ground water which moved through fault conduits. Fracturing is not evident in the sand layers. Fracture spacing is not evident in the sand layers. Fracture spacing in the sandstone ranges from six inches (6") to ten feet (10') or more. Some minor amounts of poorly to well-sorted, light gray to tan sand with minor gravel and claystone layer are locally present.

Undifferentiated Basalt:

A volcanic assemblage of tuffs, basalts and ash occurs beneath and interbedded with the Terteling Springs Formation. The low shear strength of portions of this unit are often associated with unstable slopes in the Boise foothills. The unit often contains expansive clays. The volcanic assemblages are identified by dark brown soils with mud- cracked and fissured surfaces.

Intracanyon Terrace Gravel:

Fluvial sand and sandy gravel with subangular cobbles and boulders are characteristic of this lithologic unit. These deposits form terraces along some of the gulches and also flat divides between canyon/gulch near the margins of the foothill-valley environment. Induration and stratification are relatively poor. These deposits represent stream and wave cut action of the Dry Creek drainage.

Quaternary Alluvial Fan Deposits:

This unit is comprised of light gray to tan sand and gravel deposits. The sand and gravel is poorly indurated and poorly stratified. Fracturing is not evident. The deposits are usually located near the mouth of established drainages.

Landslide Deposits:

This unit is comprised of unsorted, unstratified rock and soil masses which have been deposited by slumps, landslides, and debris flows. The particle size may range from clay to large boulders. The ground surfaces are hummocky with flat lobate toes. This unit also includes the scar area from which the soil and rock mass originated and displaced rock units which are not related to faulting. The color is dependent upon the source material. Fractures are not evident. Slope failure is evident in the study area, particularly north of Current Creek.

Recent Alluvium Deposits:

This lithologic unit consists of unconsolidated silt, sand and gravel deposits of the Dry Creek drainage, Currant Creek, and McFarland Creek. Fractures are not evident. This unit also includes recent flood plain deposits along Dry Creek.

   Figure 2-E

(Ord. 1002, 10-28-2025)