The Carnegie Ridge near 86⁰ W. : structure, sedimentation and near bottom observations Public Deposited

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  • 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.
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