1999 Field Trip, Lafayette Geological Society - A professional organization for earth scientists

DeGray Spillway

This section exposes a 1000' vertical succession of steeply dipping turbidite strata in the middle Jackfork Group. These strata record repeated cycles of turbidite sedimentation in a deep marine basin. The rocks are exposed along two spillway walls that are approximately 300' apart. The overall vertical sequence grades upward from thin-bedded sandstone and shale at the base, through thicker-bedded sandstone, into massive pebbly sandstone and conglomerate at the top. Major cycles of sandstone deposition are separated by 10' - 30' thick shales of hemipelagic or debris flow origin.
Turbidites are the result of a type of gravity sedimentation. The classic turbidite sequence, the Bouma Sequence, is defined as "A fixed, characteristic succession, of five intervals, that make up a complete sequence of a turbidite. One or more of the intervals may be missing. The five intervals, from top: E) pelitic; D) upper parallel laminations; C) current ripple laminations; B) lower parallel laminations; A) graded." [Glossary of Geology, 4^th Edition] Sediments in the Jackfork turbidites are generally fine grained.

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De Gray Dam Spillway section
from Brian E. Lock, 1978 SACGS guidebook

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Chaotic deposit separates two major cycles of sandstone deposition. Each bed represents a separate depositional episode.
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Brian gives us an idea of the thickness of this chaotic deposit.

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West wall--an olistostrome showing a large sandstone intrabasinal clast in mudstone.

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Thin beds of sandstone and shale. Typically, the uppermost beds in a package are thinner and show flat bases and tops.

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Sam and Ron inspect massive sandstone and conglomerate beds near the top of the section.
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Slump fold in sandstone bed.

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Graham inspects poorly sorted pebbly sandstone bed,
a granule conglomerate.

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Undulating contact produced by soft sediment deformation and foundering of one bed into underlying, liquefied beds.

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Sole marks at the base of a graded bed-- indicate scour of the mud bottom produced by 'tools' (clasts)
carried by the flow, followed immediately by deposition.

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Ripple marks--symmetry may indicate formation in deep water?

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Cross bedded sandstone.
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Bryan and Graham ponder the thick, imbricated sandstone.

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Graham and Ron stand by the contact between sandstone beds while we look.

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Web Author: Karen W. Broussard

	Click for larger view. Chaotic deposit separates two major cycles of sandstone deposition. Each bed represents a separate depositional episode.	Click for larger view. Brian gives us an idea of the thickness of this chaotic deposit.
	Click for larger view. West wall--an olistostrome showing a large sandstone intrabasinal clast in mudstone.	Click for larger view. Thin beds of sandstone and shale. Typically, the uppermost beds in a package are thinner and show flat bases and tops.
	Click for larger view. Sam and Ron inspect massive sandstone and conglomerate beds near the top of the section.	Click for larger view. Slump fold in sandstone bed.
	Click for larger view. Graham inspects poorly sorted pebbly sandstone bed, a granule conglomerate.	Click for larger view. Undulating contact produced by soft sediment deformation and foundering of one bed into underlying, liquefied beds.
	Click for larger view. Sole marks at the base of a graded bed-- indicate scour of the mud bottom produced by 'tools' (clasts) carried by the flow, followed immediately by deposition.	Click for larger view. Ripple marks--symmetry may indicate formation in deep water?
	Click for larger view. Cross bedded sandstone.	Click for larger view. Bryan and Graham ponder the thick, imbricated sandstone.