A diamictite dichotomy: Glacial conveyor belts and olistostromes in the Neoproterozoic of Death Valley, California, USA

Abstract

Multiple intercalations of glacially derived and slope-derived diamictites testify to the drawbacks of correlating Neoproterozoic diamictites more widely, but shed new light on the close interrelationship of these processes in the Cryogenian world. In the Neoproterozoic of Death Valley, California (USA), rifting of Rodinia occurred concomitantly with a major glacial event that deposited the Kingston Peak Formation. A new sedimentologic investigation of this formation in the Silurian Hills demonstrates, for the first time, that some diamictites are ultimately of glacial origin. Abundant dropstone textures occur in interstratified heterolithic deposits, with clasts of identical composition (gneiss, schist, granite, metabasite, quartzite) to those of boulder-bearing diamictites suggesting a common source (the glacial conveyor belt). In stark contrast, megaclast-bearing diamictites, yielding clasts of carbonate and siliciclastic preglacial strata as much as 100 m across, are interpreted as olistostromes. The occurrence of syn-sedimentary faults within the succession allows glacial versus slope-derived material to be distinguished for the first time.

A slippery slope for Cryogenian diamictites?

“…Despite the operation of comparable depositional processes in the Saddle Peak and Alexander Hills, coupled with apparently identical stratigraphic context and position, caution should be employed against any assumption that the two were deposited coevally. The idea that megaclast‐bearing diamictites are locally derived and result from carbonate platform collapse, perhaps involving major faults in the Alexander Hills, has previously been suggested in the Kingston Range (Walker et al., ) and Silurian Hills (Le Heron et al., ). Moreover, it is widely accepted that Precambrian syn‐sedimentary faults within the Kingston Peak Formation record the rifting of Rodinia (Le Heron et al., ; Mahon, Dehler, Link, Karlstrom, & Gehrels, ; Macdonald et al., ; Prave, ; Walker et al., ).…”

“…The idea that megaclast‐bearing diamictites are locally derived and result from carbonate platform collapse, perhaps involving major faults in the Alexander Hills, has previously been suggested in the Kingston Range (Walker et al., ) and Silurian Hills (Le Heron et al., ). Moreover, it is widely accepted that Precambrian syn‐sedimentary faults within the Kingston Peak Formation record the rifting of Rodinia (Le Heron et al., ; Mahon, Dehler, Link, Karlstrom, & Gehrels, ; Macdonald et al., ; Prave, ; Walker et al., ). During rifting, time‐transgressive evolution of fault systems (Alves et al., ; Eyles & Januszczak, ) would be expected at the margins of the Death Valley basin, and thus correlating deposits laid down in the lee of degrading fault scarps is fraught with difficulty.…”

“…The southernmost Kingston Range exposes by far the thickest Kingston Peak Formation outcrop in the entire Death Valley area, and would presumably therefore be expected to contain the most complete sedimentary archive (Le Heron et al., ). Despite this, the Silurian Hills succession is thinner yet has four olistostrome intervals, generally <300 m thick, with megaclasts of both carbonate and metabasite (Le Heron et al., ). Regionally, the different number of olistostrome intervals strongly suggests that basin‐bounding faults operated diachronously from outcrop belt to outcrop belt, undermining their use as chronostratigraphic markers.…”

“…This is especially true for Cryogenian diamictites of Death Valley, which often record coeval glaciation and rifting (Eyles & Januszczak, ; Miller, ; Prave, ; Vandyk et al., ). In the Silurian Hills outcrop belt (Figure ), Le Heron, Tofaif, Vandyk, and Ali () proposed that diamictites ultimately of glacial origin were characteristically far travelled, typically comprising well rounded clasts from basement lithologies. This argument hinged on dropstones being compositionally identical to clasts within associated glaciogenic diamictites.…”

New perspectives on the Luoquan Glaciation (Ediacaran‐Cambrian) of North China

“…Two other glaciations (Kaigas, ca 757–741 Ma, and Gaskiers, ca 590–582 Ma) are also recognized but considered to represent non‐Snowball Earth conditions (Gao et al., ; Li et al., ; Pu et al., ). In the absence of other age constraints, diamictites of presumed glacial origin are widely used in the correlation of glacial cycles and palaeoclimate in the Neoproterozoic (Eyles & Eyles, ; Le Heron, Tofaif, Vandyk, & Ali, ). The Gaskiers glaciation of north east North America is an example of an increasingly tightly constrained glaciation, likely ≤340 Ka in duration, which was posited to have occurred <9.5 Myr prior to the advent of the Ediacaran biota (Pu et al., ).…”