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.
The distinction between diamictites ultimately of glacial origin and those of mass flows derived from slope collapse is of fundamental paleoclimatic importance and has remained a prominent part of the research agenda in Neoproterozoic studies for over 50 yr (e.g., Schermerhorn, 1974; Eyles and Januszczak, 2004; Domack and Hoffman, 2011; Nascimento et al., 2016). Misinterpreting slope deposits as glacial, or vice versa, means that the number of glacial cycles in Earth’s history could be either under- or overemphasized (Arnaud and Etienne, 2011). Such an understanding is of profound importance in the Cryogenian, with putative panglacial (sensuHoffman, 2009) episodes in the Neoproterozoic ∼720–635 m.y. ago (Spence et al., 2016). In Death Valley (California, USA), the Kingston Peak Formation (KPF) was deposited in a regional sediment trap that received sediment from multiple directions (Mahon et al., 2014). A glacial influence has long been suspected (Hazzard, 1939). Mid-oceanic ridge basalt (MORB)–type pillow lavas on the western Death Valley flank (Labotka et al., 1980; Miller, 1985), en echelon growth faults in the Kingston Range (Walker et al., 1986), major lateral thickness variations (Prave, 1999), and olistostrome complexes (Le Heron et al., 2014) all underscore the view that the sediments record glaciation superimposed on a rift event, or vice versa (Mrofka and Kennedy, 2011; Petterson et al., 2011). The Silurian Hills (SH) are one of the Death Valley outcrop belts that have received very little study in the past 40 yr. Their exposures yield critical insight into the competing influence of rift and glacial process in the Cryogenian. In the SH, Basse (1978) entertained two working hypotheses for the diamictite-bearing strata. The first was a mass-flow origin, supported by co-occurrence with repetitive turbidite deposits in the succession. The second was the contention that the deposits were glaciomarine, but the absence of strong indicators for glacial deposition made this “inconclusive” (Basse, 1978, p. 51). In this paper, we resolve this controversy, with widespread implications for how rift-related and glacially related diamictites can be distinguished in the Cryogenian. The sections occur near the westward limit of the Basin and Range province and are cut through by a series of fault systems in multiple orientations that include Miocene extensional overprint of older fault zones (Ferrill et al., 2012). In spite of the rocks having locally reached amphibolite grade (Kupfer, 1960), delicate sedimentary textures are well preserved throughout the succession.
A new geological facies map of the KPF in the SH (Fig. 1) was completed during two field seasons over 2015 and 2016. Most lithological boundaries on Kupfer’s (1960) map are extremely accurate, and thus our mapping focused on documenting the context of megaclasts (sensuTerry and Goff, 2014), their relationship to faults, and diamictite units. Broadly, Kupfer’s (1960) mapping units p8–p11 correspond to preglacial strata of the Beck Spring–Horse Thief Spring undifferentiated unit (see Mahon et al.  for the latter unit) on our map. By comparison, what we recognize as the KPF encompasses units p12–p19 of Kupfer (1960) and corresponds to division II of Basse (1978), while the Noonday Dolomite (representing the well-established postglacial cap carbonate unit elsewhere in Death Valley; Creveling et al., 2016) corresponds to unit p20 of Kupfer (1960). Detailed measured sections (drawn to a resolution of 10 cm) underpin the simplified sedimentary log herein, which demonstrates an almost 1.4-km-thick succession (Fig. 2) (see the GSA Data Repository1). Not all diamictites are mappable; hence, the greater number of these are shown on the simplified sedimentary log than on the map.
Lonestone-Bearing Heterolithics Facies Association
The lonestone-bearing heterolithics facies association comprises normally graded sandstones and mudrocks, sandwiched between diamictite intervals (Figs. 1 and 2). Lonestones in these strata have not historically been interpreted as dropstones: “the penetrative deformation criterion is not satisfied” (Basse, 1978, p. 51). However, careful reinvestigation by the authors found several examples of lonestones that do deflect and penetrate delicate underlying laminations and cross-laminae, and these are overlain by undeformed laminae. These are spread throughout the succession, first appearing 6 m above the base of the KPF (Fig. 3A), last appearing in the top 30 m of the formation (Fig. 3B), and appearing at multiple intervals in between (Fig. 2). They include gneiss, schist, granite, and quartzite pebbles and cobbles.
Boulder-Bearing Diamictite Facies Association
The boulder-bearing diamictite facies association comprises structureless clast-poor to clast-rich muddy diamictites. Clasts are subrounded to rounded and gneissose, schistose (Fig. 3C), granitic, or quartzitic in composition. In stratigraphic section, uninterrupted 100-m-thick intervals of diamictite are recognized (Fig. 2). In some cases, diamictites show no trends in matrix character over such intervals, whereas in others, upward-coarsening and rare upward-fining patterns can be observed. At the meter scale, intercalation of diamictite beds with heterolithic, graded and rippled sandstones occurs (Figs. 2 and 3D). Most observed beds are massive, though rare occurrences of poorly stratified diamictites with wispy lamination (discontinuous, undulose bed partings) are noted.
Megaclast Diamictite Facies Association
Appraisal of both the map (Fig. 1) and the log (Fig. 2) shows that the megaclast diamictite facies association occurs at multiple stratigraphic levels. These intervals are typically >100 m each in thickness; some are fault-bounded (Fig. 1; see the Data Repository). They are characterized by megaclasts of orange dolostone (Fig. 3E), sandstone, and limestone, set within a black-weathering, locally folded (Fig. 3F) diamictite matrix. Rarely, above metabasite “sills” (Kupfer, 1960; Basse, 1978), clasts of metabasite of identical character occur within the megaclast diamictite facies association (Fig. 3G). Megaclasts are angular and measure up to 100 m in length (Fig. 1). Gneiss, schist, granite, and quartzite pebbles and cobbles are present, but only within intraclasts of the boulder-bearing diamictite facies association. At Rattlesnake Ridge, bed thicknesses change across a series of normal faults; these faults are sealed by a thick package of the megaclast diamictite (Figs. 3H and 3H′; see also the Data Repository). This includes the largest megaclast in the SH (Figs. 1, 3H, and 3H′); previously interpreted as a Cambrian klippe (Basse, 1978), it is compositionally identical to orange dolostone and limestone clasts elsewhere in the succession.
DATA INTERPRETATION AND MODEL
The observation that gneiss, schist, granite, and quartzite clasts penetrate, puncture, and deform underlying laminae suggests that these clasts are dropstones. Moreover, the intercalation of these with delicately laminated and rippled deposits indicates that the clasts are hydrodynamically incongruent (i.e., were deposited by a different process than the encasing laminae; Le Heron, 2015). Multiple ice-rafted debris (IRD) intervals are thus recognized (Fig. 4). These textures, coupled with the graded beds described in detail by Basse (1978) and interpreted as turbidites, testify to a subaqueous origin (Fig. 4). Allied to this, the boulder-bearing diamictite facies association is not considered to record primary ice-contact sedimentation (i.e., tillites), but rather debris-flow deposits. This interpretation carries an important caveat: the compositional similarity of the clasts in these debrites to the dropstones, in tandem with their intercalation with IRD, strongly supports the suggestion that they are glaciogenic debris-flow deposits (GDFs) ultimately sourced from an ice sheet or glacier. While no striations on clasts are reported, comparison to a near-identical facies association some 30 km to the north at Sperry Wash (Busfield and Le Heron, 2016) shows that where the metamorphic grade is lower, delicate striations occur.
We offer a very different interpretation for the megaclast diamictite facies association. The highly angular nature of the megaclasts, predominantly composed of preglacial sedimentary material, suggests a local source and minimal transport. The assemblage bears close similarity to the olistostrome complex widely reported from the Kingston Range (Macdonald et al., 2013; Le Heron et al., 2014), and a similar mechanism is proposed here. The field evidence points to fault arrays that were active during deposition because thickness changes occur across the faults, andsome fault arrays are sharply overlain by megaclast diamictites, some of which are themselves fault bounded (Figs. 1, 3H, and 3H′; see also the Data Repository). Therefore, unroofing and toppling of material from footwall blocks in the hinterland may well account for the size, shape, and composition of olistoliths (Fig. 4). This interpretation is strengthened by the concentration of metabasite clasts immediately above metabasite bodies, implying reworking of extruded igneous materials (rather than sill bodies as previously described; Kupfer, 1960) during olistostrome emplacement. These interpretations suggest that both glacial and slope processes vied for prominence in the sedimentary record of the KPF in the SH. Evidence for ice rafting is unequivocal, and the compositional similarity between clasts as IRD and clasts in the boulder-bearing diamictite facies association suggests that the glacial conveyor belt was dominated by crystalline basement clasts.
IMPLICATIONS AND CONCLUSIONS
Based on the data herein, it is proposed that at least four principal pulses of boulder-bearing diamictite—which we interpret as GDFs—represent multiple phases of sediment release from the paleo–ice margin. Dovetailed with observations in the southern Kingston Range some 30 km to the east (Le Heron et al., 2014), this suggests that the glacial record is much more complicated, with potentially many more glacial phases recognized, than the popular two-phase model for Death Valley which seeks to incorporate putative Sturtian and Marinoan panglacials (Macdonald et al., 2013; Smith et al., 2016).
In addition to establishing the case for Neoproterozoic glaciation in the SH for the first time, the intercalation between olistostromes and GDFs illustrates the competition between these processes in building the Cryogenian record of Death Valley. They also underscore local sensitivity to tectonic processes: the stratigraphic contrast with one olistostrome complex sandwiched between two diamictites in the Kingston Range depocenter (Macdonald et al., 2013) is striking. Given that the global age constraints for Cryogenian glaciation are permissive of a continuous Neoproterozoic record (Spence et al., 2016) and that the KPF is still undated (see Smith et al., 2016), we speculate that the diamictites are diachronous from outcrop belt to outcrop belt within the Death Valley area (cf. Prave, 1999). A pulsed record of intercalated GDFs and olistostrome deposits—the diamictite dichotomy—records a highly complex interplay of glacially related and rift-related sedimentation. Being from one of Death Valley’s thickest occurrences of the KPF (Prave, 1999), which hence potentially records one of the greatest number of sedimentary events in the basin, the data also revive the possibility that the sedimentary record actually records many more than the two glacial episodes that are popularly suggested (e.g., Kennedy et al., 1998; Spence et al., 2016).
Le Heron thanks Tony Prave for suggesting the SH as a study area and for a continually collegial attitude to the Death Valley geology, and thanks the Geological Society of London for multiple small grants that made this work possible. We are also very grateful to Débora B. Nascimento and Gene Domack for reviewing the manuscript, and to Judith Totman Parrish for her editorial work.