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Contribution 1103, Hawaii Institute of Geophysics

Dredge samples collected from the continental slope off northern Chile and central to northern Peru elucidate the late Cenozoic geologic history of forearc. Late Miocene siltstone, claystone, and sandstone were recovered in water depths ranging from 2,779 to 8,132 m from the steep slope off Chile. Manganese slabs and crusts on these rocks and the virtual absence of Quaternary sediment cover suggest that this slope is a region of nondeposition.

In marked contrast, fine-grained carbonate rocks were recovered from rock outcrops from 3.5°S to 12°S latitude off Peru in water depths ranging from 202 to 3,749 m. They are dated from late Miocene to middle Pleistocene by floral, faunal, and radiometric techniques. These carbonates are classified as calcareous siltstone, micrite, glauconitic micrite, dolomicrite, and brecciated dolomicrite. The latter two lithologies predominate in all dredges and commonly contain more than 80% dolomite with minor calcite by weight percent. These carbonates are overlain by late Pleistocene and Holocene deposits that consist largely of terrigenous mud.

Microfossil data show that the nearly pure carbonate material was deposited in open marine conditions similar to those found in the upwelling regime on the Peru Margin today. Benthic microfossils indicate that the carbonates in the Lima Basin at 12°S were originally deposited in water depths of 150–500 m, but they were dredged from minimum water depths ranging from 1,639 m to 837 m. We conclude that the seward flank of the basin has subsided 1,100 m since the Pliocene and the landward flank 500 m during the late Pleistocene. The carbonate deposits are estimated to be the dominant lithology in the upper 1.3 km of a 2 km thick sedimentary section from a multichannel seismic section nearby.

Stable isotopic C13 and O18 values (PDB standard) suggest that the Peru carbonates are organically derived. The δO18 values are well constrained between +5.00 and +7.080/oo, while the δC13 values range widely between +19.63 and —13.47 0/00. We propose that oxidation of the organic carbon within the upwelling sediments is the most likely carbonate source for the dolomites. Progressive fractionation of C13 during the formation of CO2 within a relatively closed system may be involved to produce the wide range of δC13 values, which far exceed the δC13 values of shallow-water, deep-water, and evaporitic carbonates. The Peru carbonates apparently formed by direct precipitation as pore space cement or by diagenetic alteration of the existing terrigenous sediments.

In middle Miocene time, the Amazon River drainage shifted from the Pacific to the Atlantic Ocean, greatly reducing the terrigenous sediment input to the Peru margin. Dolomitic strata appear to be prolific following this event, although wholly terrigenous muds began to accumulate on the slope between 0.93 to 0.44 m.y. ago.

The source of magnesium in the dolomicrites is speculative. Magnesium may be diagenetically released to interstitial waters from the tests of benthic foraminifera containing Mg-calcite. Alternatively, magnesium may be derived from the marine waters of restricted marginal seas created by a combination of postdepositional uplift of structural features on the upper slope and the late Miocene and Pliocene eustatic lowerings of sea level. The Pliocene and late Pleistocene subsidence of the slope structures returned the Lima Basin to open marine conditions, and the late Pleistocene terrigenous muds began to accumulate. The apparent lack of dolomitization in these organic-rich muds is difficult to explain without invoking these tectonic processes.

The brecciated dolomicrites and faulted structures in seismic reflection records indicate a complicated deformation history, especially at 8° and 9°S latitude. Dating of the breccia blocks and carbonate cement show that the deformation of some late Miocene to late Pliocene carbonates occurred in late Pliocene to early Pleistocene time. High grain densities and low porosities combined to produce high compressional velocities (4.5 to 6.6 km/sec), especially in the brecciated rocks.

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