Impact of differential tectonic subsidence on isolated carbonate-platform evolution: Triassic of the Nanpanjiang Basin, south China

Abstract

The Nanpanjiang Basin of south China contains four exceptionally well-exposed, isolated Triassic carbonate platforms. Detailed mapping of two-dimensional transects and description of stratigraphic sections allow the reconstruction of facies architecture, sequence stratigraphy, and evolution of the platforms. Biostratigraphy, magneticsusceptibility profiles, and volcanic-ash horizons allow chronostratigraphic correlation and, thus, a basinwide evaluation of mechanisms controlling platform evolution.A comparison of platform architecture demonstrates that southerly platforms have substantially greater thickness, backstepping geometry, pinnacle development, and earlier drowning that resulted from greater tectonic subsidence proximal to a probable convergent margin along the southern perimeter of the basin. Felsic volcanics thicken southward and contributed to the termination of the southernmost platform, indicating the development of a volcanic arc along the southern margin of the South China tectonic block. The northernmost isolated platform had greater longevity and lesser accumulation and lacks backstepping and pinnacle phases of development. Basin-margin intertonguing relationships, or lack thereof, demonstrate that earlier siliciclastic influx into the basin to the south and concurrent starved-basin conditions to the north impacted the evolution of platform-margin geometries. GEOLOGIC NOTEComparative analysis of platform evolution shows that the timing and rates of tectonic subsidence controlled the timing of platform termination by drowning, backstep geometries, pinnacle development, and overall platform thickness. The timing of siliciclastic basin fill dictated differences in platform-margin geometries such as slope angle, relief above basin floor, and the presence or absence of basinward platform progradation. Despite the dramatic differences in platform architecture, eustatic sea level fluctuations imparted a basinwide sequence-stratigraphic signal.

An integrated biostratigraphy (conodonts and foraminifers) and chronostratigraphy (paleomagnetic reversals, magnetic susceptibility, elemental chemistry, carbon isotopes and geochronology) for the Permian–Upper Triassic strata of Guandao section, Nanpanjiang Basin, south China

“…Six new single-zircon analyses from sample PGD Tuff-1 that occurs 4.20 m below the O-A boundary in the lower Guandao section (Lehrmann et al, , 2007a(Lehrmann et al, , 2007b form a highly coherent cluster with a weighted mean 206 Pb/ 238 U date of 247.458 ±0.053/0.12/0.29 Ma (see Table S1 for notation) and mean square of weighted deviates (MSWD) of 0.55 (Fig. 8).…”

“…Sample PGD Tuff-3 from 4.90 m above the O-A boundary in the lower Guandao section (Lehrmann et al, , 2007a(Lehrmann et al, , 2007b produced six new zircon analyses, four of which form a coherent cluster with a weighted mean 206 Pb/ 238 U date of 247.08 ± 0.11/0.17/0.31 Ma (Table S1 and Fig. 8) and a MSWD of 0.39.…”

Depositional setting and basin development of the Paleocene—Lower Eocene Sinjar and Khurmala formations, Northern Iraq

“…(Lehrmann et al 2007;Enos et al 2006;Li et al 2013). Li et al (2013) summarized the major factors that affect the geometry of carbonate platform margins and the evolution of margin architecture over time, namely, rate of production of carbonate sediment and the degree of stability of the platform margin and slope.…”

“…In addition, the carbonate platform asymmetry and architecture are influenced by hydrodynamic windward-leeward effect (Bergman et al 2010) and tectonic effects, antecedent topography, or asymmetries in basin sediment fill patterns (Lehrmann et al 2007). The studied Paleocene-Lower Eocene successions recorded foreland sedimentation developed during the Alpine orogeny.…”

Anachronistic facies in the Lower Triassic of South China and their implications to the ecosystems during the recovery time

“…It was bordered on the north by the Qinling-Dabie Orogenic Belt, a suture between the North China and South China blocks [36][37][38] . To the northwest, it bordered the SongpanGanzê Fold System, interpreted to be a remnant oceanic basin filled with Triassic flysch [39] .…”