A JURASSIC SPRING-FED LAKE SYSTEM IN THE BRUSHY BASIN MEMBER OF THE MORRISON FORMATION, TEN MILE GRABEN, UTAH, USA; BIOSIGNATURE PRESERVATION AND ASTROBIOLOGICAL IMPLICATIONS

The upper part of the Jurassic Brushy Basin Member of the Morrison Formation in Ten Mile Graben, southern Utah hosts a <10m thick carbonate hypothesized to represent a small spring-fed lake. This deposit has the potential to host ancient biosignatures and microbial mat features typically not preserved in the rock record and provides a unique opportunity to identify these features in an ancient spring deposit. This lake system was northwest of neighboring Lake T’oo’dichi’, an alkaline saline, siliciclastic deposit that has been proposed as an analog to the rocks at Gale crater, Mars. Carbonates have been observed in the Brushy Basin, but most carbonates are usually observed as thinly bedded and laterally restricted micritic limestones or limestone nodules. This previously unreported carbonate presented here suggests that this lake system to the north hosted a different water chemistry than Lake T’oo’dichi’.

The extent of the carbonate deposit at Ten Mile Graben is > 8km² and the unit is 6.0m at its thickest. The carbonate layer is underlain by 30.0m of variegated smectitic clays and tuff. Beds of silcretes (> 0.5m thick) were found emplaced in between some of the mudstone layers within 10m of the carbonate. A 1.5m thick microbiolite layer overlies the carbonate in some areas of Ten Mile Graben. At one location, there is a greater siliciclastic input and a greater abundance of silcrete beds. Chert veins and wavy bedding are also present in the carbonate layer suggesting the potential source of groundwater input. The carbonate layer transitions laterally into what appears to be shoreline sands. Alternating sequences of fine laminations and bioturbated sands (also known as lam-scram) were observed in the sandstone which is interpreted as a paleoshoreline along the ancient lake.

Comparison of the Jurassic carbonate to more modern analogs (progressively older tufas ranging from 1-400ka) present in Ten Mile Graben shows that the major commonality between these springs is the presence of delicate terrace textures. Delicate textures have also been preserved in the Brushy Basin carbonates which are at least 145Ma. The springs that were 300 kyr and 400 kyr did not have terrace textures, but the absence of textures is likely due to intense erosion.