Late-stage diagenetic concretions in the Murray formation, Gale crater, Mars

Abstract

Concretions are prevalent features in the generally lacustrine deposits of the Murray formation in Gale crater. In this work, we document the morphologic, textural, and chemical properties of these concretions throughout 300 meters of Murray formation stratigraphy from Mars Science Laboratory observations between Sols 750-1900. We interpret these observations to constrain the timing and composition of post-depositional fluid events at Gale crater. We determine that the overall diversity of concretion morphology, size, texture, and chemistry throughout the Murray formation indicates that concretions formed in multiple, likely late diagenetic, episodes with varying fluid chemistries. Four major concretion assemblages are observed at distinct stratigraphic intervals and approximately correlate with major distinct chemical enrichments in Mg-S-Ni-Cl, Mn-P, and Ca-S, among other local enrichments. Different concretion size populations and complex relationships between concretions and veins also suggest multiple precipitation events at Gale crater. Many concretions likely formed during late diagenesis after sediment compaction and lithification, based on observations of concretions preserving primary host rock laminations without differential compaction. An upsection decrease in overall concretion size corresponds to an inferred upsection decrease in porosity and permeability, thus constraining concretion formation as postdating fluid events that produced initial cementation and porosity loss. The combined observations of late diagenetic concretions and distinct chemical enrichments related to concretions allow constraints to be placed on the chemistry of late stage fluids at Gale crater. Collectively, concretion observations from this work and previous studies of other diagenetic features (veins, alteration halos) suggest at least six post-depositional events that occurred at Gale crater after the deposition of the Murray formation.

Mineral‐Filled Fractures as Indicators of Multigenerational Fluid Flow in the Pahrump Hills Member of the Murray Formation, Gale Crater, Mars

“…Similar gray material is observed in the lower portions of the Pahrump Hills stratigraphic column, particularly around the Chinle outcrop, where resistant ridges along vertical fractures grade into adjacent bedding planes and laminations . Gray ridges also share a chemical similarity to other diagenetic features near the base of Pahrump Hills such as dendritic crystal clusters that are enriched in magnesium Nachon et al, 2016;Sun et al, 2019). Such spatial and chemical similarity to other diagenetic features supports an interpretation that gray material was deposited relatively early in the diagenetic sequence, prior to complete lithification and loss of porosity in the Murray formation host rock.…”

“…Chemical similarities between gray veins and other diagenetic features at Pahrump Hills, including crystal clusters and dendrites, suggest that they were deposited relatively early in the diagenetic sequence and preceded the formation of white Ca-sulfate veins Sun et al, 2019). Veins at Pahrump Hills exhibit characteristically thicker, more complex morphologies than those previously observed by the Curiosity rover.…”

“…Gray ridges also share a chemical similarity to other diagenetic features near the base of Pahrump Hills such as dendritic crystal clusters that are enriched in magnesium Nachon et al, 2016;Sun et al, 2019). Similar gray material is observed in the lower portions of the Pahrump Hills stratigraphic column, particularly around the Chinle outcrop, where resistant ridges along vertical fractures grade into adjacent bedding planes and laminations .…”

“…Relatively early in the diagenetic sequence, magnesium-rich fluids permeated the Murray formation, depositing material along fractures (gray veins) and within pore space as clusters and dendrites Sun et al, 2019). Relatively early in the diagenetic sequence, magnesium-rich fluids permeated the Murray formation, depositing material along fractures (gray veins) and within pore space as clusters and dendrites Sun et al, 2019).…”

“…Ehlmann & Edwards, 2014;Grotzinger & Milliken, 2012;McLennan et al, 2005;Thomas et al, 2007). At the scale of rover observations, diagenetic features provide unambiguous evidence for postdepositional fluid flow and, in Gale crater, include concretions Sun et al, 2019), dendritic crystal clusters Nachon et al, 2016), crystal pseudomorphs , veins Nachon et al, 2014Nachon et al, , 2016, and fracture-associated alteration halos (Frydenvang et al, 2017;Yen et al, 2017). Despite the pervasiveness of diagenetic features within the rocks of Gale crater, the relative timing of postdepositional fluid events is poorly constrained, as are their formation mechanisms.…”