The Carnegie Ridge is a linear, aseismic, submarine ridge
lying between the Galapagos Islands and the coast of South America.
A 2300 meter deep saddle near 86°W. longitude divides the ridge into
western and eastern segments. Surface ship, near bottom, and grain
size studies from the saddle have been used to delineate the present
geological environment and history of the ridge.
Structurally the Carnegie Ridge is rather simple in profile,
being bounded by east-west trending scarps which give the ridge a
block-faulted appearance. Acoustic basement over the ridge appears
smooth on reflection profiles and is composed of chert. The sedimentary
sequence above the chert horizon contains a lower chalk unit
overlain by calcareous ooze. Where erosion has exposed the chalk a
karst-like micro-topography is present which is characterized by
steep walled channels and cliffs and consolidated bed forms undergoing
erosion and dissolution.
The ridge crest has been stripped of almost its entire sediment
cover. Thick sequences of sediment are found only in areas protected
from north or south flowing bottom currents. Evidence of
erosion is provided by extensive channeling on both the north and
south flanks of the ridge. Near bottom observations in one channel on
the north flank revealed a large field of sand dunes indicating northward,
downslope sediment transport. These dunes are found on a
manganese-encrusted chalk which floors the channel. Hydrographic
data suggest that the northward flow across the ridge may be produced
by the spillover of bottom water. Near bottom and surface ship
observations are consistent with a southward sediment transport on
the south flank of the ridge. The mechanism responsible for this
southward flow remains unresolved. Current meters deployed on the
north and south flanks recorded only low speed currents, opposite in
direction to the inferred sediment transport. Apparently the bottom
water flow responsible for erosion and sediment transport over the
ridge is episodic in nature and was not recorded during the present
survey.
The grain size characteristics of surface sediments respond to
the same processes which control sediment distribution. Where
erosion is evident over the ridge crest, coarse lag deposits of foraminiferal sand are found. Apparently the erosion is most pronounced
at the sill depth on the ridge since the sediments tend to become finer
both upslope and downslope from that point. Three dominant modes
are present in the sand fraction from the ridge. These modes record
the initial input and fragmentation of foraminiferal tests. Continued
fragmentation and dissolution of these tests creates a large number
of finer modes.
The age of true basaltic crust over the ridge is between 10 and
26 million years. This crust was probably created during a period of
very slow spreading on the Galapagos Rift Zone during the Miocene.
Unconformities on the ridge indicate that erosion dates only from the
mid-Pliocene. The initiation of erosion was probably in response to
further uplift of the ridge. This uplift may have been related to slight
southward underthrusting along the north flank of the ridge. / Graduation date: 1975 / Best scan available for figures on p.67, 96. The original is a black and white photocopy.
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28552 |
| Date | 04 September 1974 |
| Creators | Malfait, Bruce Terry |
| Contributors | van Andel, T. H. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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