Geology and Soil
Pouhala Marsh is located on recent sediments deposited on the southern flank of the Ko‘olau Volcano as shown in Figure 2‑7. The Ko‘olau lavas are divided into Ko‘olau Basalt and Honolulu Volcanics. There are no Honolulu Volcanics in the vicinity of Pouhala. The Ko‘olau Basalt primarily consists of Pliocene aged shield stage tholeiitic basalt (Lagenheim and Clague 1987). Holocene and Pleistocene sedimentary caprock deposits directly underlay the project area.
The rocks of the Ko‘olau Basalt can be divided into three groups, lava flows (a‘a and pahoehoe), pyroclastic deposits, and dikes. The lava flows of the Ko‘olau basalt are usually thin bedded with an average thickness of about ten feet (Wentworth and MacDonald 1953). These beds are composed of a‘a and pahoehoe flows and pyroclastic deposits. A‘a contains a solid central core between two gravely clinker layers. Pahoehoe flows are usually characterized by a smooth ropy texture. Pyroclastic deposits originate from explosive volcanism. They are composed of friable sand-like ash and indurated tuff deposits. Dikes are thin near vertical sheets of rock that intruded or squeezed into existing lava flows or pyroclastic deposits.
The caprock is composed of a wedge of a diverse variety of terrestrial and marine sediments. It forms a coastal plain about 5,000 feet wide between basalt outcrops in the Waipahu and Pearl Harbor coast. The caprock is slightly over 100 feet thick in Pouhala (Wentworth 1951). The caprock in the project area has been covered with artificial fill. The surficial deposits in the project area are predominantly mud, soil, and pebbles deposited by Waikele Stream. The artificial fill in the area is composed of saprolitic red soil.
The Natural Resources Conservation Service (NRCS) classifies the soils in Pouhala as mixed artificial fill as shown in Figure 2-8. Waipahu Silty Clay and Tropaquepts soils are also found adjacent to the project area. Waipahu Silty Clay weathered directly from the existing volcanic rock and is common near the ocean in southern O‘ahu. Tropaquepts are soils formed in wetlands. They were formed in conditions of periodic flooding.
Figure 2-7. Geology Map of Pouhala Marsh
Figure 2-8. Soils Map of Pouhala Marsh