LITHOSPHERIC STRENGTH: A POTENTIAL CONTROLLING FACTOR OVER DIFFERENTIAL SUBSIDENCE IN THE EARLY TRIASSIC SONOMA FORELAND BASIN (WESTERN USA)?

The Sonoma Foreland Basin was formed as a flexural response to the emplacement of the Golconda Allochthon during the Sonoma Orogeny around the Permian/Triassic boundary (~252 Ma), and it includes an excellent fossil and sedimentary record of the Early Triassic. However, sediments deposited from the Permian/Triassic boundary until the end-Smithian (~250.7 Ma) shows marked thickness variations between its southern (up to ~200 m thick) and northern (up to ~550 m thick) parts. Differences in dominant lithologies can also be noticed throughout the basin, with mostly terrigenous sandstones and microbial limestones in the southern part while the northern part is dominantly composed of fine siltstones and carbonates.

 

Based on an integrated study within a high-resolution timeframe and using georeferenced data from sedimentological, paleontological, structural and cartographic approaches, we propose to discuss the controlling factor(s) responsible for spatial variations in sedimentary thickness. Regional palinspastic reconstruction and high time-resolution subsidence analyses are thus conducted to obtain an accurate image of the basin by the time of the deposition.

 

We notably show that sedimentary overload in the basin and heterogeneity of the topographic load exerted by the Golconda Allochthon, when emplaced, are not sufficient to explain the significant discrepancy observed within the Early Triassic sedimentary record. We therefore argue that the age and thus, the rheological properties of the terranes that compose the basin basement are of paramount importance in controlling both the subsidence and thickness spatial distribution across the Sonoma Foreland Basin.