Yellowstone Lake

Otis et al. (1977) surveyed the Yellowstone Lake part of the caldera in 1973-74 with a 1-in.3 air gun providing 350 ms/ sec penetration and with a 7-kHz system for the surface layer. This procedure generated a ba-thymetric and a sediment thickness map (190 m maximum), as well as evidence of a deep turbidite basin along paleodrainage. They documented faulting, hydrothermal activity, and a variety of glacial features. In 1992, 3.5-kHz profiling was used to target 10-m-long Kullenberg cores on deep bathymetric highs to collect undisturbed, mainly pelagic sediment sequences without turbidites for a diatom evolution study.

The profile FD between Frank and Dot islands (Fig.

9) crosses the late glacial/Holocene lithology at Core Site FD-2 (Tiller and Kelts, in press). This lithology, with tephra and accelerator mass spectrometer (AMS) radiocarbon age correlations, shows a major change from clastic, bedded mud at ca. 10,600 14C B.P. (-8 m) to lam-

FIGURE 9 3.5-kHz profile section from Yellowstone Lake between Dot and Frank Islands showing core site of 9-m-long YEL-92-FD2 on a rise about 8 m above the 70-m deep plain, where the reflection package thins. Reflection Y divides ponded glacial deposits and postglacial muds from a pelagic Holocene diatom-ooze sequence.

FIGURE 9 3.5-kHz profile section from Yellowstone Lake between Dot and Frank Islands showing core site of 9-m-long YEL-92-FD2 on a rise about 8 m above the 70-m deep plain, where the reflection package thins. Reflection Y divides ponded glacial deposits and postglacial muds from a pelagic Holocene diatom-ooze sequence.

inated Holocene diatom muds (—7.5 m) and then burrow mottled to faintly banded, uniform, Holocene diatom ooze from ca. 6500 14C B.P. (—5 m) to the present. The parallel-bedded acoustic character of the Pg reflector series is consistent with an interpretation of ponded proglacial turbidites such as were just touched by the corer. The Y reflection at the base of the nearly transparent Holocene package is correlated with two, decimeter-thick, bluish-gray clay sediment layers believed to be ca. 10,000 14C years old and derived from the last phreatic eruption into the lake. The Holocene (h series reflections) has a typical pelagic acoustic signature that drapes over the rise. Several weak internal reflectors occur, but do not correlate clearly with litho-logical features.

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