Stable isotope characterization of pedogenic and lacustrine carbonates from the Chinese Tian Shan: Constraints on the Mesozoic–Lower Cenozoic palaeoenvironmental evolution

Abstract

In the Mesozoic-Cenozoic continental deposits of the Tian Shan area, two main levels containing 17 pedogenic carbonates have been identified on both the southern and northern foothills of the range: 18 one in the Upper Jurassic series and one in the Upper Cretaceous-Lower Palaeocene series. In order 19 to reconstruct the palaeo-environmental and palaeo-topographic characteristics of the Tian Shan 20 area during these two periods, we measured the oxygen and carbon isotope composition of these 21 pedogenic carbonates (calcrete and nodules). The stable isotope compositions are homogeneous: 22 most  18 O values are between 21 and 25‰ and most  13 C values are between-4 and-6‰. No 23 distinction can be made between the calcrete and nodule isotopic compositions. The constancy of 24 isotopic values across the Tian Shan is evidence of a development of these calcification features in 25 similar palaeo-environmental conditions. The main inference is that no significant relief existed in 26 that area at the Cretaceous-Palaeogene boundary, implying that most of the present relief 27 developed later, during the Cenozoic. In addition to the pedogenic carbonates, few beds of 28 limestones interstratified in the Jurassic series of the southern foothills display oxygen and carbon 29 Manuscript 2 isotope compositions typical of lacustrine carbonates, ruling out brackish water incursion at that 30 period in the region. 31 32 Keywords: pedogenic carbonates-stable isotopes-Mesozoic palaeo-environment-Tian Shan 33 34 1. Introduction 35 Calcification is a ubiquitous phenomenon that occurs in a large variety of geological settings 36 including limestone diagenesis and continental weathering. In continental environments, under semi-37 arid conditions, the interaction between meteoric waters or groundwaters and sedimentary rocks 38 commonly leads to pedogenic calcification (e.g. Wright and Tucker, 1991; Alonso-Zarza, 2003; 39 Hasiotis et al., 2007). The resulting calcareous features correspond to the so-called calcretes and 40 occur as isolated nodules or rather continuous levels of carbonates (Retallack, 1997; Wright, 2008). 41 The oxygen and carbon isotope composition of such neo-formed carbonates provides information on 42 the palaeo-environments within which they precipitated (e.g. Alonso-Zarza, 2003). Indeed, while the 43 oxygen isotope composition of these carbonates depends upon processes taking place in the soil-44 ground water system, such as evaporation, it is also function of the composition of the meteoric 45 water. The later depends on the palaeo-latitude and the palaeo-altitude of the area, as well as on the 46 distance to the ocean from which the atmospheric masses are derived. In a specific region, any 47

Differential Exhumation and Crustal Tilting in the Easternmost Tianshan (Xinjiang, China), Revealed by Low‐Temperature Thermochronology

“…Sedimentological records from the northern Tarim Basin and the southern Junggar Basin show increased rates of basin subsidence during the Late Cretaceous and early Paleogene, accompanied by the deposition of coarse clastic sediments (Hendrix et al, ). Following the localized deformation and uplift described above, the area once again experienced erosion and planation, forming a new regional erosional surface in western Mongolia and the Chinese Tianshan (Cunningham et al, ; Devyatkin, ; Heilbronn et al, ).…”

“…Cunningham et al () furthermore noted the presence of a Late Cretaceous to early Cenozoic planation surface preserved in many parts of the study area, prominently on the southern slopes of Harlik Shan and on more spatially limited areas of Balikun Shan. Planation surfaces of this age are also preserved in western Mongolia and in other parts of the Chinese Tianshan (Devyatkin, ; Heilbronn et al, ). This planation surface provides important constraints on the interpretation of thermochronological data from the study area.…”

Planation surfaces of the Tian Shan Range (Central Asia): Insight on several 100 million years of topographic evolution

“…Lower Cretaceous fine-grained alluvial to lacustrine sediments were subsequently deposited within both basins (Hendrix et al, 1992;Shao et al, 1999), before being unconformably overlaid by Upper Cretaceous alluvial fan deposits (Hendrix et al, 1992;Shao et al, 1999). Along the southern margin of the Junggar Basin, the Paleogene sediments then contain thick calcrete layers (Heilbronn et al, 2015).…”

Denudation intensity and control in the Chinese Tian Shan: new constraints from mass balance on catchment‐alluvial fan systems

“…; Heilbronn et al. ), which form under semi‐arid to semi‐humid climate with less than 1000 mm/yr of precipitation (Retallack et al , , and references therein). This could imply that the Chinese Tian Shan has been experiencing a low denudation rate for a few tens of millions of years, which is in agreement with geological observations such as preserved Mesozoic surfaces in the mountain range and a limited Cenozoic exhumation (Hendrix et al.…”

“…This figure is available in colour online at www.interscience.wiley.com/journal/espl. deposits of the northern piedmont contain pedogenic carbonates (Charreau et al, 2012;Heilbronn et al, 2015), which form under semi-arid to semi-humid climate with less than 1000 mm/yr of precipitation (Retallack et al, 1997, and references therein). This could imply that the Chinese Tian Shan has been experiencing a low denudation rate for a few tens of millions of years, which is in agreement with geological observations such as preserved Mesozoic surfaces in the mountain range and a limited Cenozoic exhumation (Hendrix et al, 1994;Dumitru et al, 2001;Jolivet et al, 2010).…”