The top of the section also has basalt flows and diatomite as well.
The rock in question is diatomite, made of countless silica skeletons of microscopic aquatic plants.
Deposits of diatomite are prominently exposed at the surface in the southwestern portion of the subbasin.
But the formation of a diatomite deposit in a freshwater lake requires that many processes take place in and adjacent to the lake.
British Dictionary definitions for diatomite
a soft very fine-grained whitish rock consisting of the siliceous remains of diatoms deposited in the ocean or in ponds or lakes. It is used as an absorbent, filtering medium, insulator, filler, etc See also diatomaceous earth
an unconsolidated form of diatomite Also called kieselguhr
diatomaceous earth di·a·to·ma·ceous earth (dī'ə-tə-mā'shəs, dī-āt'ə-) n. A powder made of the desiccated shells of diatoms, used as a filtering agent, adsorbent, and abrasive in many chemical operations.
(dī-āt'ə-mīt') A fine, light-colored, friable sedimentary rock consisting mainly of the silica-rich cell walls of diatoms. Diatomite forms both in lacustrine and marine environments. It is used in industry as a filler, filtering agent, absorbent, abrasive, and insulator.