Kenneth L. Finger

PALEONTOLOGIC EVIDENCE FOR SEDIMENTARY DISPLACEMENT IN NEOGENE FOREARC BASINS OF CENTRAL CHILE

Abstract A consensus on the biostratigraphic age and depositional environment of the Navidad, Ranquil, and Lacui formations exposed along the tectonic margin of central Chile has been elusive due to conflicting evidence. This study resolves this dilemma and gains further insight regarding the history of the Chilean coast. Problematic interpretations stem primarily from the remarkable similarity between the molluscan fauna of these units with those well documented for the late Oligocene to early Miocene of Peru. Planktic foraminifers, however, indicate that the Chilean sections accumulated in the late Miocene to early Pliocene interval following a regional hiatus that extends into the Eocene. The prevalence of mixed-depth bathyal assemblages of benthic foraminifers and ostracodes, the majority of which include lower-bathyal (>2000 m) indicators, reveals that downslope displacement was a primary mode of deposition in the basins. Although the molluscan assemblages are dominated by shallow marine taxa, most include species that range into or are restricted to deeper waters. Sedimentary features connote rapid subsidence and deep-water deposition of gravity flows. Although older Tertiary and Cretaceous planktic foraminifers in several assemblages indicate reworking of older units, lack of data on pre-Tortonian faunas of this region precludes recognition of other age-discordant components that could constitute a significant portion of the recovered fauna. The findings of this study revise the prevailing conception of the regions geologic history that considered these units to be early to middle Miocene shelf deposits and indicate that infilling and uplift have characterized the nearshore basins since the late Pliocene.

High-resolution strontium-isotope stratigraphy and biostratigraphy of the Miocene Monterey Formation, Central California

Detailed biostratigraphic and Sr-isotope studies on two outcrop sections of the Miocene Monterey Formation of California demonstrate the applicability of the Sr-isotope method for detailed chronostratigraphic correlation. Multidisciplinary biostratigraphic ages generally agree with Sr isotopic ages determined by correlating 87Sr/86Sr ratios with a standard 87Sr/86Sr versus age curve constructed of data from Deep Sea Drilling Project Sites 575 and 590. Diagenetic modification of Sr isotopic ratios of the Monterey Formation was negligible. Strontium isotopes provide age resolutions of 0.1 to 0.2 Ma in the lower Miocene. Small faults and slump structures are identified as reversals within a general trend of increasing 87Sr/86Sr. Biostratigraphy yields the more definitive age assignments in the younger siliceous rocks where diatoms in particular have more utility than the corresponding flat and polytonal portion of the isotopic curve. Strontium isotopic correlation of the basal Monterey Formation with Deep Sea Drilling Project Site 575 indicates that Monterey deposition commenced at 17.85 ± 0.10 Ma (within planktic foraminiferal zone N6 and calcareous nannofossil zone CN2), concomitant with an increase in slope of the sea water 87Sr/86Sr versus age curve. This event dates 0.35 ± 0.10 m.y. before the beginning of the pronounced shift of δ13C, and 1.4 m.y. before the shift of δ18O, observed at Site 575. It is evident that Sr-isotope stratigraphy is a valuable supplement to biostratigraphy; the techniques are complementary in refining chronostratigraphic interpretations.

Major forearc subsidence and deep-marine Miocene sedimentation in the present Coastal Cordillera and Longitudinal Depression of south-central Chile (38°30'S-41°45'S)

Neogene marine strata crop out in the present Coastal Cordillera and Longitudinal Depression of south-central Chile between 38°30′S and 41°45′S, indicating the onset of a major marine transgression that covered most of the forearc in this area. In order to determine the sedimentary environment, paleobathymetry, and age of these deposits, we carried out integrated sedimentologic, ichnologic, and micropaleontologic studies on samples from oil wells and outcrops in the region. Our results indicate that these successions were deposited at lower-bathyal depths (>2000 m) during the middle to late Miocene. Shallow-marine deposition followed in the southwestern part of the study area during the Pliocene(?). We attribute deep-marine sedimentation in this area to a major event of subsidence in the Miocene that affected the entire forearc and that was caused by basal subduction erosion. We suggest that the anomalously thin crust that characterizes this area may have facilitated forearc subsidence and allowed the Miocene transgression to advance much farther inland here than in other regions of Chile. Subsequent uplift of the forearc is ascribed to basal accretion or underplating of sediments. Our conclusions contradict previous studies that favor a stable margin at these latitudes since the Jurassic. Deep-marine sedimentation in this area during the Miocene implies that the present Coastal Cordillera and Longitudinal Depression were probably submerged during that epoch.

Late Oligocene–early Miocene submarine volcanism and deep-marine sedimentation in an extensional basin of southern Chile: Implications for the tectonic development of the North Patagonian Andes

The Chilean margin has been used as the model of an ocean-continent convergent system dominated by compression and active mountain building as a consequence of the strong mechanical coupling between the upper and the lower plates. The Andean Cordillera, however, shows evidence of alternating phases of compressional and extensional deformation. Volcano-sedimentary marine strata in the Aysen region of southern Chile contribute to an understanding of the causes of extensional tectonics and crustal thinning that occurred in the Andean orogeny because these deposits constitute the only reliable record of submarine suprasubduction volcanism during the Cenozoic in southern South America. In order to discern the age and tectono-sedimentary setting of these strata, referred to as the Traiguen Formation, we integrated sedimentology, ichnology, petrography, geochemistry, structural geology, foraminiferal micropaleontology, and U-Pb geochronology. Our results indicate that the Traiguen Formation was deposited in a deep-marine extensional basin during the late Oligocene–earliest Miocene. The geochemistry and petrography of the pillow basalts suggest that they formed in a convergent margin on a thinned crust rather than at an oceanic spreading center. We attribute the origin of the Traiguen Basin to a transient period of slab rollback and vigorous asthenospheric wedge circulation that was caused by an increase in trench-normal convergence rate at ca. 26–28 Ma and that resulted in a regional event of extension and widespread volcanism.

Implications of Deep-Marine Miocene Deposits on the Evolution of the North Patagonian Andes

The North Patagonian Andes (38°–43°30′S) present topographic and geologic characteristics distinct from those of the Central Andes to the north, including a lower altitude, a predominance of plutonic rocks, and the presence of a major N-S-trending, dextral strike-slip, intra-arc discontinuity known as the Liquiñe-Ofqui Fault Zone (LOFZ). The timing for the uplift of the North Patagonian Andes remains poorly constrained principally because high exhumation rates resulted in the erosion of most of its volcano-sedimentary cover during the late Cenozoic. The strongly deformed Cenozoic marine deposits ascribed to the Ayacara Formation, which crop out on the western flank of this range at ∼42°S, can provide for a better understanding of the recent geologic evolution of this range. To discern this formation’s age and sedimentary environment, we have integrated sedimentology, ichnology, foraminiferal paleontology, and geochronology. Our results indicate that this unit was deposited in a deep-marine environment during the Early-Middle Miocene. Correlation of the Ayacara Formation with coeval deep-marine deposits in the forearc to the west indicate that deposition was caused by a major regional event of tectonic subsidence probably related to subduction erosion. An Early-Middle Miocene age for the Ayacara Formation is a reliable maximum age for the deformation and uplift of the western flank of the North Patagonian Andes as well as for the commencement of transpressional deformation associated with the LOFZ.

Stratigraphic Implications of Latest Middle Miocene to Earliest Late Miocene Diatoms in the Navidad Formation at Lo Abarca, Central Chile (33° 30'S)

Neogene marine strata crop out at various localities along the Chilean coastline (see Encinas et al., 2008 and references therein). The best studied of these successions occurs in the Navidad area, between San Antonio and Punta Topocalma (33°30’34°30’S) (figure 1). Named and described by Darwin (1846) as the Navidad Formation, the geology and paleontology of this unit was subsequently studied by many other authors (e.g., Philippi, 1887; Brüggen, 1950; García, 1968; Tavera, 1979) and has been considered the stratigraphic reference for the marine Neogene of Chile (e.g., Cecioni, 1980; De Vries and Frassinetti, 2003). However, its age, stratigraphy and depositional environment were a matter of debate for several decades (e.g., Brüggen, 1950; García, 1968; Tavera, 1979; Encinas et al., 2006). Recent foraminiferal and sedimentologic studies indicate a late Miocene to early Pliocene age and a deep-marine setting for this unit (Finger et al., 2007; Encinas et al., 2008). Yet, the older latest Oligocene to middle Miocene age proposed for the molluscan fauna of this unit (De Vries and Frassinetti, 2003) still constitutes an unresolved problem (see below). North of the Navidad area, between Valparaíso and San Antonio (33°00’?33°30’S), Neogene successions have been traditionally referred to as the “capas de Lo Abarca” (Covacevich and Frassinetti, 1990) (figure 1). These strata have been considered equivalent to the Navidad Formation by some authors (Brüggen, 1950; Fuenzalida and Varela 1964; Encinas et al., 2006) and as a different unit by others (Martínez-Pardo and Parada, 1968; Covacevich and Frassinetti 1990). The main argument for distinguishing the successions is based on their different molluscan faunas (Covacevich and Frassinetti, 1990). Nevertheless, Encinas et al. (2006) considered both successions as corrrelative and included the Lo Abarca strata in the Navidad Formation because their diatoms and foraminifera indicate similar ages and both units show similar facies. Essential to the age constraint of the Lo Abarca succession and its correlation with the Navidad Formation is the diatom assemblage found at Lo Abarca section that we document in this note.

Ostracod Assemblages in the Frasassi Caves and Adjacent Sulfidic Spring and Sentino River in the Northeastern Apennines of Italy

Rich, diverse assemblages comprising a total (live + dead) of twenty-one ostracod species belonging to fifteen genera were recovered from phreatic waters of the hypogenic Frasassi Cave system and the adjacent Frasassi sulfidic spring and Sentino River in the Marche region of the northeastern Apennines of Italy. Specimens were recovered from ten sites, eight of which were in the phreatic waters of the cave system and sampled at different times of the year over a period of five years. Approximately 6900 specimens were recovered, the vast majority of which were disarticulated valves; live ostracods were also collected. The most abundant species in the sulfidic spring and Sentino River were Prionocypris zenkeri, Herpetocypris chevreuxi, and Cypridopsis vidua, while the phreatic waters of the cave system were dominated by two putatively new stygobitic species of Mixtacandona and Pseudolimnocythere and a species that was also abundant in the sulfidic spring, Fabaeformiscandona ex gr. F. fabaeformis. Pseudocandona ex gr. P. eremita, likely another new stygobitic species, is recorded for the first time in Italy. The relatively high diversity of the ostracod assemblages at Frasassi could be attributed to the heterogeneity of groundwater and associated habitats or to niche partitioning promoted by the creation of a chemoautotrophic ecosystem based on sulfur-oxidizing bacteria. Other possible factors are the geologic age and hydrologic conditions of the cave and karst aquifer system that possibly originated in the early– middle Pleistocene when topographic uplift and incision enabled deep sulfidic waters to reach the local carbonate aquifer. Flooding or active migration would have introduced the invertebrates that now inhabit the Frasassi Cave system.

The Late Oligocene–Early Miocene Marine Transgression of Patagonia

The most important Cenozoic marine transgression in Patagonia occurred during the late Oligocene–early Miocene when marine waters of Pacific and Atlantic origin flooded most of southern South America including the present Patagonian Andes between ~41° and 47° S. The age, correlation, and tectonic setting of the different marine formations deposited during this period are debated. However, recent studies based principally on U–Pb geochronology and Sr isotope stratigraphy, indicate that all of these units had accumulated during the late Oligocene–early Miocene. The marine transgression flooded a vast part of southern South America and, according to paleontological data, probably allowed for the first time in the history of this area a transient connection between the Pacific and Atlantic oceans. Marine deposition started in the late Oligocene–earliest Miocene (~26–23 Ma) and was probably caused by a regional event of extension related to major plate reorganization in the Southeast Pacific. Progressive extension and crustal thinning allowed a generalized marine flooding of Patagonia that reached its maximum extension at ~20 Ma. It was followed by a phase of compressive tectonics that started around 19–16 Ma and led to the growth of the Patagonian Andes. The youngest (~19–15 Ma) marine deposits that accumulated in the eastern Andean Cordillera and the extra-Andean regions are coeval with fluvial synorogenic deposits and probably had accumulated under a compressive regime.

Slow rates of subduction erosion and coastal underplating along the Andean margin of Chile and Peru: COMMENT and REPLY COMMENT

In their article, [Clift and Hartley (2007)][1] propose the existence of two alternating modes of subduction erosion for north-central Chile and Peru: one fast, with steady-state retreat from 150 to 20 Ma, and the other slow, with erosion constrained only in the trench domain from 20 Ma onward. The

Tectonic Implications of Deep-Marine Miocene Strata in the Western Andean Cordillera of South–Central Chile (40°–42°S)

Marine Cenozoic strata crop out in the western flank of the North Patagonian Andes at Lago Ranco (40°S) and Ayacara (42°S). These deposits, known as the Estratos de Lago Ranco and Ayacara formations, consist of rhythmic successions of sandstone and siltstone. In order to unravel the sedimentary environment, age, and tectonic history of this area during the Neogene, we conducted sedimentological, ichnological, and micropalaeontological studies. In addition, we dated detrital zircons by U–Pb (LAICPMS and SHRIMP). Our results show that these strata were deposited in a deep-marine environment during the early–middle Miocene. Our findings indicate that the uplift of the western flank of the North Patagonian Andes took place after the early–middle Miocene.