Tracking Exhumation of Andean Ranges Bounding the Middle Magdalena Valley Basin, Colombia

Tracking exhumation of Andean ranges bounding the Middle Magdalena Valley Basin, Colombia

Junsheng Nie1,2, Brian K. Horton1,3, Andrés Mora4, Joel E. Saylor1, Todd B. Housh1, Jorge Rubiano4, and Julian Naranjo4 1Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA 2 National Laboratory of Western China’s Environmental Systems, Ministry of Education, Lanzhou University, Lanzhou, Gansu 730000, China 3Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA 4Instituto Colombiano del Petróleo, Ecopetrol, Bucaramanga, Colombia

ABSTRACT ous rocks (Aspden and McCourt, 1986; Aspden The shortening history of the Andes is important for understanding retroarc deformation et al., 1987; Gómez et al., 2005). The Eastern along convergent margins and forcing mechanisms of Cenozoic climate. However, the tim- Cordillera has continental-affi nity basement ing of uplift in the northern Andes is poorly constrained, with estimates ranging from Cre- of Proterozoic–Paleozoic age that is mostly taceous to Pliocene. Detrital zircon U-Pb ages from the Middle Magdalena Valley Basin in covered by Paleozoic–Mesozoic strata (Dengo Colombia reveal two provenance shifts during Cenozoic time. The fi rst shift occurs between and Covey, 1993; Gómez et al., 2005; Chew et early and late Paleocene strata, where U-Pb results show a switch from Proterozoic-dom- al., 2007). To the east of these ranges and the inated to Phanerozoic-dominated age spectra. We attribute this change to uplift-related Llanos foreland basin is the Amazonian craton, exhumation of the Central Cordillera. The second shift occurs between middle-late Eocene consisting of several terranes accreted to an and late Oligocene strata, where increased Grenville ages and diminished Mesozoic ages can Archean nucleus. be linked to uplift of the Eastern Cordillera. Our results show that signifi cant pre-Neogene Cenozoic strata in the Middle Magdalena deformation affected the northern Andes, underscoring the potential importance of Andean Valley Basin consist of the Lisama, La Paz, uplift on the dynamics of Paleogene climate. Esmeraldas, Mugrosa, Colorado, Real, and Mesa Formations. Ages of Paleocene–middle INTRODUCTION Cenozoic accretion (McCourt et al., 1984; Miocene units are based on palynology and Despite decades of research, the forcing Dengo and Covey, 1993). invertebrate fossils (Hopping, 1967; Nuttall, mechanisms for Cenozoic cooling remain elu- Detrital zircon U-Pb age analyses have 1990; Gómez et al., 2005). Ages for late Mio- sive (Zachos et al., 2001). Intensifi ed chemi- proven useful in defi ning the provenance his- cene–Pliocene units are based on intercalated cal weathering associated with tectonic uplift tory of sedimentary basins and tracking exhu- tuffs (Gómez et al., 2005). (Raymo and Ruddiman, 1992) has been sug- mation of sediment source regions. In this study The Paleocene Lisama Formation transition- gested as the most plausible cause for the cool- we evaluate erosional exhumation of the Central ally overlies the Maastrichtian shallow-marine ing trend in Cenozoic climate. Previous efforts Cordillera and Eastern Cordillera using detri- Umir Formation and records regressive sedi- have focused on the role of plateau uplift in Asia tal zircon U-Pb geochronology from Cenozoic mentation in deltaic and alluvial plains (Gómez (Raymo and Ruddiman, 1992; Garzione, 2008). strata in the intermontane Middle Magdalena et al., 2005). The middle to late Eocene La Paz However, South America has the highest non- Valley Basin of the Colombian Andes (Fig. 1). Formation consists of amalgamated channel collisional plateau and longest latitudinal range Potential source regions (Horton et al., 2010; sandstones and minor mudstones that uncon- of Earth’s mountain belts; if proven to have initi- see the GSA Data Repository1), including the formably overlie the Lisama Formation. Eocene ated during the Paleogene, the Andes could be Western Cordillera, Central Cordillera, Eastern strata were deposited in a fl uvial setting and indi- an important force for Cenozoic climate cool- Cordillera, and Amazonian craton, have diag- cate transformation of the Middle Magdalena ing. Although pre-Neogene shortening and nostic basement ages, allowing us to recognize Valley Basin into a nonmarine basin (Gómez uplift has been proposed for the central Andes provenance shifts recorded by sediments of the et al., 2005). The early Oligocene Esmeraldas based on integrated stratigraphic and structural Middle Magdalena Valley Basin (Fig. 2). Formation is also composed of broadly lenticu- considerations (Horton et al., 2001; DeCelles lar fl uvial sandstones, but with a much higher and Horton, 2003; Horton, 2005; McQuarrie GEOLOGIC SETTING proportion of fi ne-grained overbank deposits. In et al., 2005), the uplift history of the northern Three ranges in the Colombian Andes (West- the late Oligocene Mugrosa Formation, abun- Andes remains poorly understood. For the East- ern Cordillera, Central Cordillera, Eastern Cor- dant well-developed pedogenic features have ern Cordillera of Colombia, initial uplift ages of dillera) are separated by the Magdalena and overprinted channel sandstones and fl oodplain latest Cretaceous, middle Eocene, late Eocene– Cauca valleys (Fig. 1). The Western Cordillera mudstones. Available paleocurrent data indicate early Oligocene, and middle Miocene–Pliocene is composed of Late Cretaceous–Cenozoic dominantly eastward fl ow in the La Paz and (Dengo and Covey, 1993; Hoorn et al., 1995; igneous rocks of oceanic affi nity (McCourt et Esmeraldas Formations (Gómez et al., 2005); Gregory-Wodzicki, 2000; Gómez et al., 2003; al., 1984; Aspden and McCourt, 1986). The the Mugrosa Formation mainly shows a strike- Bayona et al., 2008; Parra et al., 2009; Horton Central Cordillera has mixed continental and perpendicular, westward or eastward direction. et al., 2010) have all been proposed using strati- oceanic basement that is intruded and over- The contact with the early–middle Miocene graphic and thermochronological data. Esti- lapped by numerous Jurassic–Cretaceous igne- Colorado Formation is transitional, with con- mates for initial uplift of the Central Cordillera, tinued evidence for extensive pedogenesis and based primarily on sedimentological evidence, 1GSA Data Repository item 2010122, review of upward coarsening to alluvial-fan boulder con- range from mid-Cretaceous to early Cenozoic basement ages of potential source regions, supple- glomerates at the top. The late Miocene Real (Cooper et al., 1995; Villamil, 1999; Gómez mentary method, and U-Pb geochronologic analy- Formation is distinguished from the Colorado ses, is available online at www.geosociety.org/pubs/ et al., 2005; Jaimes and de Freitas, 2006). For ft2010.htm, or on request from editing@geosociety Formation by a greater proportion of volcanic the Western Cordillera, available studies sug- .org or Documents Secretary, GSA, P.O. Box 9140, and igneous detritus, along with northward- gest initial uplift during Late Cretaceous–early Boulder, CO 80301, USA. directed paleocurrents (Gómez et al., 2005).

© 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, May May 2010; 2010 v. 38; no. 5; p. 451–454; doi: 10.1130/G30775.1; 2 ﬁ gures; Data Repository item 2010122. 451

Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/38/5/451/3539099/451.pdf by guest on 02 October 2021 Figure 1. A: Map showing tectonomorphic provinces of northern Andes. B: Simpliﬁ ed geologic map of Middle Magdalena Valley Basin (MMVB). C: Simpliﬁ ed cross section across MMVB. S—Santander massif; P—Panama arc; WC—Western Cordillera; CC—Central Cordillera; EC—Eastern Cordillera.

METHODS age probability diagram and age histogram for Oligocene−late Miocene record. For the upper Medium- and coarse-grained sandstones each sample (Fig. 2). Following previous stud- Mugrosa (U08025, late Oligocene), lower Colo- were collected from the eastern limb of the ies, age peaks on age probability diagrams are rado (M09, early Miocene), upper Colorado Nuevo Mundo syncline, a type locality for considered signifi cant only if defi ned by three (U08027, middle Miocene), and lower Real Cenozoic fi ll of the Middle Magdalena Val- or more analyses (e.g., Dickinson and Gehrels, (U08028; late Miocene) samples, (1) Grenville ley Basin (Gómez et al., 2005) (Fig. 1C). We 2008). This approach minimizes the likelihood ages (1200–900 Ma) become more abundant obtained eight samples from fi ve exposed for- of erroneously identifying source terranes based and (2) Late Jurassic–Early Cretaceous ages mations (Fig. 1C). Detrital zircon grains were on ages affected by Pb loss in young grains or (150–100 Ma) are eliminated. separated by standard heavy liquid techniques, discordance in older grains. selected randomly, and analyzed by laser- DISCUSSION AND CONCLUSIONS ablation–inductively coupled plasma–mass RESULTS Detrital zircon U-Pb ages from the Middle spectrometry in the Department of Geological The lower Lisama sample (RS0114091, early Magdalena Valley Basin reveal two age popula- Sciences at the University of Texas at Austin. Paleocene) has no signifi cant zircon age popula- tion shifts during the Cenozoic. The fi rst occurs Analyses and associated age calculations fol- tions younger than 500 Ma; instead, most ages between early and late Paleocene strata, where lowed methods outlined in the Data Repository, are concentrated at 2000–1500 Ma and 1000– U-Pb results indicate a change from Protero- utilizing results for zircon standards Plesovice 500 Ma. In contrast, the overlying upper Lisama zoic-dominated to Phanerozoic-dominated age (337.13 ± 0.37 Ma; Slama et al., 2008) and in- (U821, late Paleocene), lower La Paz (U08022, spectra. The second shift occurs between mid- house standard S97–19. middle Eocene), and upper La Paz (CU612P, dle-late Eocene to late Oligocene strata, where We report 730 U-Pb ages (Table DR1) for the 8 middle-late Eocene) samples show abundant Grenville ages increase markedly and Late samples, including only those analyses between Phanerozoic ages, with 2 of the 3 samples exhib- Jurassic–Early Cretaceous ages are eliminated. 40% and −20% discordant for ages older than iting 150–100 Ma ages. Few signifi cant popula- Based on a review of potential sediment sources 500 Ma and between 65% and −20% discordant tions (defi ned by 3 or more grains, as observed (see the Data Repository), we attribute the fi rst for ages younger than 500 Ma. A 50% uncer- on histograms) are detected in the 2000–500 Ma shift in age spectra to a provenance change from tainty fi lter (2σ) was applied to all samples. Dis- range in these samples. the craton to the Central Cordillera and the sec- cordance was calculated based on 206Pb/238U and In comparison to the Paleocene–Eocene ond age shift to a provenance change from the 207Pb/235U ages. Results are plotted on a relative samples, two changes are detected in the late Central Cordillera to the Eastern Cordillera.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/38/5/451/3539099/451.pdf by guest on 02 October 2021 whereas the increased amount of Grenville-aged zircons is consistent with an Eastern Cordillera source. A probable source of Grenville aged zircons is the Cretaceous section of the Eastern Cordillera (Horton et al., 2010), consistent with initial recycling of the sedimentary cover of the Eastern Cordillera. Ages of ca. 600–400 Ma and 300–200 Ma are also representative of the East- ern Cordillera and Santander massif (Horton et al., 2010). Therefore, we attribute the shift in age spectra between the La Paz and Mugrosa Formations to initial shortening-related uplift of the Eastern Cordillera between middle-late Eocene and late Oligocene time. The timing and polarity of this provenance shift matches evidence from growth strata for initial thrusting along the western fl ank of the Eastern Cordillera (Gómez et al., 2003). This study, together with previous studies from the central Andes (Horton et al., 2001; DeCelles and Horton, 2003; Horton, 2005; McQuarrie et al., 2005), suggests that nearly the entire latitudinal extent of the Andes likely underwent shortening-related uplift and exhumation in the Paleogene. Potential early Andean uplift is important because Paleogene climate underwent signifi cant changes, but the forcing mechanisms remain unclear. In theory, early Andean uplift could signifi cantly affect climate (1) by changing the location of the Intertropi- cal Convergence Zone (Rodwell and Hoskins, 2001; Takahashi and Battisti, 2007), which can affect global climate by affecting monsoons (Yancheva et al., 2007) responsible for a large Figure 2. Detrital zircon U-Pb ages for eight samples of Cenozoic strata in Middle Magdalena amount of moisture redistribution between Valley Basin. Normalized age probability plots (thick black lines) and age histograms (gray tropical and polar oceans (Emile-Geay et al., bars) are arranged in stratigraphic order. Note change in scale and bin size at 500 Ma. Hori- 2003; Nie et al., 2008); (2) by changing the zontal dashed lines mark levels where three analyses defi ne a bin. Colored vertical bands atmospheric CO concentration via intensifi ed highlight ages representative of Western Cordillera (WC), Central Cordillera (CC), Eastern 2 Cordillera (EC), and Amazonian craton. chemical weathering (Raymo and Ruddiman, 1992); and (3) by changing the distribution of surface-water salinity between the Pacifi c and We argue that separate episodes of shortening- rocks of the Central Cordillera (150–100 Ma). Atlantic Oceans and thus changing the pattern related uplift of the Central Cordillera and East- Because such mixed ages are not observed, of thermohaline circulation. The existence of ern Cordillera are the most plausible causes for we attribute the provenance of lower Lisama both the northern and central Andes, spanning the two provenance shifts recorded in the Mid- strata to the craton. The dominance of Late the southern tropics and equatorial zones (from dle Magdalena Valley Basin. Jurassic–Early Cretaceous ages in the over- ~25°S to 10°N), is critical in this aspect because Age spectra of the lower Lisama sample are lying upper Lisama sample (U821) suggests the belt intercepts a large amount of precipita- characterized by a lack of ages younger than that shortening-induced uplift of the Central tion on the eastern Andean slope (Horton, 1999; 500 Ma and abundant ages older than 1500 Ma. Cordillera was under way by middle Paleocene Mora et al., 2008). This process further affects Lower Lisama zircons can only originate from time. Although this shift from a craton source the tropical ocean-water salinity distribution, the Amazonian craton because limited ages to Central Cordillera source could also be which is an important factor controlling pole- younger than 500 Ma suggest little or no con- expressed in other stratigraphic measures, the ward transport of warm tropical water (Knauss, tribution from the Central Cordillera and the distal facies of the Lisama Formation reveal 1996), thus regulating the amount of polar sea Eastern Cordillera (Aspden et al., 1987; Dörr no clear changes in lithofacies, paleocurrents, ice and the ability of sinking polar water to et al., 1995; Cordani et al., 2005; Mejia et al., or composition (Cooper et al., 1995; Villamil, sustain thermohaline circulation. In summary, 2006). It is conceivable that craton-derived 1999; Gómez et al., 2005). our results suggest that initial uplift of the Cen- sediments were originally deposited across the Oligocene–Miocene samples from the tral Cordillera was under way by middle Paleo- Central Cordillera and subsequently uplifted Mugrosa, Colorado, and Real Formations record cene time, and initial uplift of the Eastern Cor- and recycled into the Middle Magdalena Val- increased Grenville ages (1200–900 Ma) and dillera occurred between the middle-late Eocene ley Basin. However, if this were the case, we the elimination of 150–100 Ma ages. The lack and the late Oligocene, underscoring the poten- would expect to see a mixed age signal of both of Late Jurassic–Early Cretaceous zircons sug- tial importance of early Andean uplift on the the craton (older than 1500 Ma) and the arc gests limited input from the Central Cordillera, dynamics of Paleogene climate.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/38/5/451/3539099/451.pdf by guest on 02 October 2021 ACKNOWLEDGMENTS Sciences, v. 8, p. 187–194, doi: 10.1016/0895 McQuarrie, N., Horton, B.K., Zandt, G., Beck, S., We thank M. Parra, C. Moreno, V. Caballero, K. -9811(95)00004-Y. and DeCelles, P.G., 2005, Lithospheric evolu- Nelson, J. Knowles, D. Tovar, N. Vann, and J. Zi- Emile-Geay, J., Cane, M.A., Naik, N., Seager, R., tion of the Andean fold-thrust belt, Bolivia, and mowski for assistance with sample collection and pro- Clement, A.C., and van Geen, A., 2003, War- the origin of the central Andean plateau: Tec- cessing. D. Chew, B. Guest, an anonymous reviewer, ren revisited: Atmospheric freshwater fl uxes tonophysics, v. 399, p. 15–37, doi: 10.1016/j and editors S. Wyld and P. Cowie provided thoughtful and “Why is no deep water formed in the North .tecto.2004.12.013. reviews that improved the manuscript. 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