Pliocene Onset Research Articles

K–Ar and 40 Ar/ 39 Ar geochronology of supergene processes in the Atacama Desert, Northern Chile: tectonic and climatic relations

New K–Ar and 40 Ar/ 39 Ar data obtained for 29 samples of supergene alunite-, jarosite- and hollandite-group minerals from the Central Depression of Northern Chile document almost continuous supergene oxidation from 33 to 9 Ma. The downward migration of supergene oxidation zones is used to calculate maximum denudation rates of 16–35 m Ma −1 . These calculated rates are sensitive to pre-existing topography and show that the present relief observed in the Central Depression represents a modified Miocene landscape. The results presented here are in good agreement with previously published geochronological data for the Atacama Desert. The combination of all published data, including the results presented here, defines a continuous skewed distribution, with a maximum between 21 and 14 Ma. The relative paucity of ages greater than 30 Ma reflects the removal of supergene profiles by erosion during and immediately after the Incaic compressive deformation. From 29 to 9 Ma, supergene oxidation profiles were generated under semi-arid conditions throughout the Atacama region. Ages between 9 and 5 Ma on the northern and southern margins of the desert mark the end of supergene oxidation processes, consistent with a Pliocene onset of present-day hyperaridity.

Response of Supergene Processes to Episodic Cenozoic Uplift, PedimentErosion, and Ignimbrite Eruption in the Porphyry Copper Province of Southern Peru

The Jurassic to middle Eocene porphyry copper deposits and prospects exposed on the Pacific slopes of the central Andean Cordillera Occidental of southern Peru between latitudes 16°309 and 18° S record a protracted, ca. 30-m.y. history of supergene processes that were fundamentally controlled by the evolving local geomorphologic environment, itself a response to successive regional tectonic events, including the late Eocene Incaic, the late Oligocene to earliest Miocene Aymara, and the middle to late Miocene Quechuan events. Weathering of the porphyry centers also overlapped temporally with the local resumption of arc volcanism in southern Peru at 25.5 Ma following a 27-m.y. amagmatic interval, and supergene processes were variously interrupted or terminated by ignimbrite blanketing, although in several locations supergene profiles were preserved by such cover. The landform chronology for the area surrounding the Cuajone, Quellaveco, and Toquepala deposits (ca. 17° S) is revised and extended northwestward through field mapping to the Cerro Verde-Santa Rosa district (ca. 16° 309 S). The 40 Ar- 39 Ar incremental-heating dates of supergene alunite group minerals from the Angostura (38.1 and 38.8 Ma) and Posco (38.8 Ma) prospects and the Cerro Verde deposit (36.1–38.8 Ma) demonstrate that supergene processes were underway in the late Eocene beneath a subplanar topography resulting from uplift and erosion during the Incaic orogeny, now represented by a regional unconformity in the Cenozoic volcanic-sedimentary rock succession. Broadly contemporaneous supergene processes were probably active in the Cuajone-Quellaveco-Toquepala district. Slow erosion and the accumulation of clastic sediments through the tectonically quiescent early to mid-Oligocene are envisaged to have caused a rise in the water table and the widespread preservation of the Incaic supergene profiles. Aymara uplift subsequently led to the incision of the 23.8 to 24 Ma Altos de Camilaca and the 18.8 to 19.1 Ma Pampa Lagunas pediplains and their regional correlatives. The ensuing water-table lowering was associated with intense leaching and sulfide enrichment from the late Oligocene (24.4–28 Ma natroalunite at Cerro Verde, 26–27 Ma natroalunite at Santa Rosa, and 28.6 Ma jarosite at La Llave) to the early Miocene (23 Ma alunite and 21 Ma natroalunite at Cerro Verde, and 19.2 Ma jarosite at La Llave) and was plausibly responsible for much of the upgrading of the Cuajone and Toquepala deposits and thr Quellaveco prospect, which are intersected by both the Altos de Camilaca pediplain and erosional features representing upslope extensions of the Pampa Lagunas pediplain. The younger supergene profiles were widely superimposed on the remnants of those generated during the Incaic orogeny. Middle Miocene (<14.2 Ma biotite age) Chuntacala Formation flows protected the Cuajone supergene profile from destruction by erosion, but at 13.0 Ma interrupted supergene processes at Quellaveco. Revision of volcano-stratigraphic relationships in the latter area reveals that subsequent erosion of the Chuntacala Formation ignimbrites and part of the supergene profile took place prior to the deposition of a 10.1 Ma ash-flow tuff of the Asana Formation. Elsewhere, supergene activity persisted at the Cachuyito prospect through 11.4 Ma, and minor jarosite development occurred at least until 4.9 Ma both there and at Cerro Verde during and following the Multiple Pediment landform stage (ca. 7.9–15.0 Ma). The occurrence of relics of late Eocene alunite group minerals within considerably younger late Oligocene to late Miocene supergene alteration profiles suggests that the overall physiographic configuration of the Pacific piedmont of southern Peru remained remarkably consistent from the late Eocene to the middle Miocene. Moreover, the new age data confirm that, as in northern Chile, semiarid climatic conditions prevailed along much of the plate boundary from the mid-Eocene until the late Miocene or early Pliocene onset of hyperaridity. The local geomorphologic and volcanic conditions in southern Peru, however, conspired to generate more complex supergene profiles with lower aggregate enrichment factors relative to the strongly enriched profiles in the late Eocene to early Oligocene porphyry copper belt of northern Chile, which underwent supergene upgrading over relatively brief periods.

The paleoclimatic record provided by eolian deposition in the deep sea: The geologic history of wind

The mineral component of pelagic sediment is brought to the deep sea by transport in the wind. Extraction and analysis of this dust allows estimation of the past aridity of the eolian source region, via flux determinations, and of the intensity of the transporting winds, from grain size data. These two parameters, the grain size and mass flux of dust, vary independently. There are three significant sources of modern dust, eastern and central Asia, northwest Africa, and Arabia, all in the northern hemisphere. As the rainfall associated with the Intertropical Convergence Zone is an effective barrier to southerly transport of dust, the northern hemisphere is an order of magnitude more “dusty” than is the southern, an asymmetry that has characterized most of the Cenozoic. Eolian flux data show that most of the northern hemisphere was more arid during glacial maxima, with 3 to 5 times as much dust transported during glacial stages than during interglacials; only northwestern South America varied in the opposite sense. The periodicity of Quaternary variation in both eolian flux and eolian grain size data is strongly influenced by the Milankovitch cycles of orbital variability. Wind intensities vary on a shorter time scale than the general 100‐kyr cycles of glaciation and general aridity. Eolian grain sizes display forcing at precessional (19 and 23 kyr), tilt (41 kyr), and at approximately 30 kyr periodicities. As a result the generalization that winds are uniformly stronger during glacial times is not valid. Whole‐Cenozoic records show that the largest change in dust flux, an order of magnitude increase, occurred in the northern hemisphere and reflects continental drying associated with the late Pliocene onset of northern hemisphere glaciation. Southern hemisphere eolian records show no sign of paleoclimatic changes in the late Pliocene. The most important change in Cenozoic atmospheric circulation was a severalfold reduction in wind intensity that occurred at the time of the Paleocene‐Eocene boundary. Before then, latest Cretaceous and Paleocene winds were essentially as strong as those of the late Cenozoic. This shift appears to be one of several climatic responses to a change in the nature and amount of global heat transport at about 55 Ma.

Gradual late pliocene onset of glaciation: A deep-sea record from the Northeast Atlantic

A detailed analysis of planktonic foraminifera assemblages and benthic oxygen isotope values from the Late Pliocence of DSDP Site 548 in the Northeast Atlantic shows that the initiation of Northern Hemisphere glaciation developed gradually, through a series of cycles. These results contrast with those of other DSDP Sites which indicate constant paleoclimatic conditions preceding the sudden, step-wise, development of glaciations at 2.4–2.5 m.y. The difference between these sites and Site 548 might be explained by broad sampling interval and breaks in the sedimentary sequence which obscure the onset of Late Pliocence glaciation at those locations where it appears to occur in a step-wise fashion. The results from Site 548 indicate that the development of Northern Hemisphere glaciation could be regarded as part of a progressive climatic deterioration rather than a geologically sudden paleoclimatic event.

Provenance Evolution and Unroofing History of a Modern Arc-Continent Collision: Evidence from Petrography of Plio-pleistocene Sandstones, Eastern Taiwan

ABSTRACT Plio-Pleistocene sandstones from the Coastal Range of eastern Taiwan provide a record of shifting source areas and unroofing events in adjacent arc and accretionary-wedge terranes during the early stages of are-continent collision. In latest Miocene and early Pliocene time, arc-derived sediments were deposited in a precollisional forearc basin between the Luzon volcanic arc and the Manila Trench. Sandstones of this age are rich in zoned plagioclase, volcanic lithic fragments, and pelagic foraminifera and contain only minor quartz and sedimentary-metasedimentary lithic fragments. In contrast, lower Pliocene to lower Pleistocene sandstones are poor in feldspar and volcanic fragments and contain abundant sedimentary and metasedimentary lithic fragments. This reflects a provenance shift f om the volcanic arc to the uplifted (meta) sedimentary accretionary wedge of proto-Taiwan, which resulted from the early Pliocene onset of arc-continent collision. A refined classification of sedimentary and metasedimentary lithic fragments is introduced here and used to interpret the early evolution of the collisional orogen. Lithic fragments are rich in pelitic minerals and are divided into: 1) sedimentary (Ls) = mudstone, shale, siltstone, chert and argillite; 2) low-grade metamorphic (Lm1) = slate, slatey siltstone, and quartzite; and 3) medium-grade metamorphic (Lm2) = phyllite-schist, phyllitic quartzite and quartz-mica-albite aggregate. A good correlation is observed between these lithic fragment types and bedrock lithologies exposed in the present-day Central Range of Taiwan. Based on this correlation, the change in relative abundance of sedimentary and metasedimentary lithic fragments through time has been used to interpret an unroofing sequence for the collisional fold-and-thrust belt. Lower Pliocene Taiwan-derived sandstones are rich in sedimentary lithic fragments (Ls), which were shed from the accretionary wedge during the earliest stages of arc-continent collision. From early Pliocene to early Pleistocene time, sandstones became progressively depleted in Ls lithic fragments and enriched first in Lm1 and later in Lm2 fragments, as deeper levels in the metamorphic complex were uplifted and exposed to erosion. Regional metamorphism of biotite-grade rocks (= Lm2) occurred at about 4.5 Ma at depths of 12-15 km (Ernst 1983) and were uplifted to the surface by about 1.4 Ma (this study). These ages give an uplift rate of 4 to 5 km/m.y., which is in good agreement with independent estimates based on fission-track studies, raised coral reefs, Quaternary elevation changes, and present-day denudation rates. Thus, it appears that sandstone detrital modes of clastic orogenic sequences can be used to interpret specific aspects of unroofing and tectonic evolution in nearby active mountain belts.

Benthic foraminiferal assemblage response to the onset of northern hemisphere glaciation: Paleoenvironmental changes and species trends in the northeast Atlantic

The response of the benthic foraminiferal community in the northeast Atlantic to the Late Pliocene onset of Northern Hemisphere glaciation was investigated using closely spaced samples from DSDP-IPOD Site 548. Factor analysis of quantitative species abundance data shows that the replacement of a Globocassidulina subglobosa assemblage first by Bulimina mexicana and then by Melonis-Uvigerina dominated assemblages, accompanied the onset of glaciation. Also, the benthic assemblages became much more variable with progressive climatic deterioration. In order to examine benthic Foraminifera species responses to changing paleoenvironmental variables, measurements of planktonic foraminifera species abundances, benthic foraminifera stable isotope values and sediment grain size and composition were made. The planktonic data and oxygen isotope values were used to quantify paleoclimatic changes and changes in bottom water properties, the sediment analyses were used to quantify changes in the substrate, and a combination of planktonic assemblage data and benthic carbon isotope values were used to estimate changes in food supply (surface ocean productivity). Canonical correlation analysis of the species data and the independent paleoenvironmental indicators shows a strong relationship between planktonic assemblage diversity and the long term benthic trend of assemblage replacement. This trend could be a response to changing bottom water properties but it might also reflect increasing food supply at the sea-bed. In addition, the benthic assemblages show a strong relationship to changes in the carbonate content of the sediments. This latter fluctuates due to varying terrigenous input. Changes in substrate at Site 548 may influence the benthic foraminiferal assemblage. There remains, however, a significant fraction of species abundance variability, particularly in the Uvigerina species, that cannot be explained with the paleoenvironmental indicators used in this study.

Late Pliocene onset of glaciation: ice-rafting and diatom stratigraphy of North Pacific DSDP cores

The revised diatom biostratigraphy for the North Pacific allows the timing of ice-rafting, as recorded in Deep Sea Drilling Project cores, to be determined. Significant ice-rafting began at the time of the Matuyama-Gauss reversal boundary, about 2.48 Ma. Other bio- and lithostratigraphic data from the Pacific, Atlantic, and Arctic Oceans, and the Black Sea all indicate that northern hemispherical cooling and the accumulation of continental ice began at 2.4–2.5 Ma. We interpret all these data to indicate northern hemisphere ice-cap formation at that time. If so, the closing of the Isthmus of Panama, approximately 3.2 m.y. ago, was not the immediate cause of the Plio-Pleistocene glaciations.