Geochemical transect through a travertine mount: A detailed record of CO2-enriched fluid leakage from Late Pleistocene to present-day – Little Grand Wash fault (Utah, USA)

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Abstract

Active and fossil endogenic travertine mounts scattered along the Little Grand Wash fault are studied as records of Quaternary CO2-enriched fluid leakage. This study focusses on a particular area where a fossil mount formed in a near-surface setting by successive circulation/sealing episodes from Late Pleistocene to Mid-Holocene and where a modern surface travertine is still being formed by a CO2-enriched fluid source. The fossil mount is composed of horizontal and vertical veins whereby the vertical veins recorded numerous cycles of circulation/sealing/dissolution events and were used as conduits for the CO2-enriched fluid circulation from the depth to the surface or along sub-horizontal fractures where successive precipitation events are recorded. The modern travertine is being built at the surface by successive eruption of Crystal Geyser, an anthropic geyser active since the 1930's.

δ13C and δ18O signatures and U/Th datings, ranging from 11.5 ky till present-day allows calibrating in detail the CO2 enriched fluid leakage along a single fault segment and in a post glacial context, as last glaciations in the study area took place 15 ky ago. The dataset shows a high decrease of the oxygen stable isotope values till about 6 ky, then the variations reflect a constant range until present-day. This tends to restrain the period of local increase of the meteoric water input in the aquifer that is sourcing the CO2-enriched water.

The fossil travertine represents a 7 ky-long record of CO2 leakage above a natural reservoir, from Late Pleistocene to Mid-Holocene. The flux of CO2 leakage through time and the total escaping volume have been computed and appears to be low in comparison with an anthropogenic leak provoked, for instance, by a non-sealed well.

Introduction

Endogenic travertines are records of paleoclimatic and paleotectonic events (Altunel and Hancock, 1993, Hancock et al., 1999, Faccenna et al., 2008, Brogi et al., 2010, Capezzuoli et al., 2010, De Filippis et al., 2011, Gratier et al., 2012, Priewisch et al., 2014, Ricketts et al., 2014, Frery et al., 2015). Travertines are calcium carbonate agglomerates known to be built under near ambient conditions in continental areas (Pentecost, 2005, Capezzuoli et al., 2014). Endogenic is use to define the origin of the CO2-enriched fluids sourcing the agglomerate formation. Endogenic travertines are usually built along fault systems, where deep CO2 and/or hot water are escaping from the depth to the surface (Janssen et al., 1999, Glover and Robertson, 2003, Crossey et al., 2006, Crossey et al., 2009). Little Grand Wash present-day active travertine called Crystal Geyser is formed by cold water (∼17 °C) with a heavy carbon isotope composition (>4‰, Kampman et al., 2009). The δ13C and δ18O signature of Green River fossil travertine mounts also demonstrate an endogenic origin of the fluids (Heath, 2004, Dockrill and Shipton, 2010). U/Th dating of these mounts calcium carbonate precipitations range from 400 ky to present-day (Burnside, 2010).

Such endogenic travertines located along Little Grand Wash and Salt Wash faults have been demonstrated to be formed episodically (Shipton et al., 2004, Dockrill, 2005, Burnside, 2010, Dockrill and Shipton, 2010, Kampman et al., 2012, Burnside et al., 2013, Frery et al., 2015). Kampman et al. (2013) showed that the CO2 pulses can be related to glacial cycles.

In this contribution a detailed study of δ13C- and δ18O-signatures recorded along a travertine mount section is presented. U/Th dating and a part of the stable isotope measurements along this profile have been published in Frery et al. (2015) in order to build a conceptual model of episodic fluid circulation along an active fault. The record lasts from Late Pleistocene to present-day, providing a unique detailed record of this period of time that deserves to be closely analysed regarding i) the structure of the dated travertine veins; ii) the significance of the data regarding the Quaternary glaciation episodes on the Northwest part of the Colorado Plateau as well as iii) the calibration of CO2 leakage during this period of time.

Results from more than 300 stable isotope analyses of the carbonate veins forming the travertine are analysed through time, based on 14 U/Th datings. Stable isotope measurements are shown for the whole dated period, from the thin section scale to the outcrop scale and on present-day active travertine forming close to the studied outcrop. Vertical and horizontal veins are studied regarding their petrography and their isotopic signature evolution through time. The results are then discussed in the perspective of the Late Holocene post-glacial cycle in the region and as potential proxy to calibrate the CO2 paleo-leaky volumes along a particular segment of the fault during this period of time.

Section snippets

Location and sampling

The study focused on the so-called T1 fossil travertine mount, which is located in the footwall of a normal fault called Little Grand Wash (Fig. 1A and B). The mount is partially eroded. An accurate chronology has been obtained through 14 radiometric datings (U/Th) of 9 travertine veins sampled along a 10 m-thick profile (Fig. 1C). All samples were cleaned using a chemical pre-treatment to remove possible contamination. 234U/230Th ages were then measured with a TIMS mass spectrometer

Field observations

The fossil travertine mount reveals a complex internal structure, composed of calcium carbonate veins capped by conglomerates, i.e. pieces of the Curtis Formation outcropping in the hanging wall of the fault and a highly altered surface travertine (Fig. 1C).

Two main kinds of calcium carbonate veins can be distinguished thanks to their different orientation and internal structure, e.g. horizontal and vertical veins. The horizontal or sub horizontal veins (Fig. 2A–D) are characterized by their

A detailed geochemical record of a post glacial interval

Green River fault leakage has been studied over the past 400,000 years on 40 separated travertine mounds by Kampman et al. (2012). This present study completes this regional work with a detailed dataset focussed on the record of Late Pleistocene to present-day CO2-enriched fluid flow. The studied T1 travertine mount records carbonate precipitations from 11.469 ± 0.015 ky ago up to 4.559 ± 0.023 ky ago, thus archiving up to 7 ky of precipitation history at this location.

During this period,

Conclusion

The studied endogenic travertine, located in the footwall of Little Grand Wash Fault, along the NW part of the Colorado Plateau, recorded CO2-enriched fluid leakage from Late Pleistocene till Mid Holocene. The detailed δ13C and δ18O dataset along T1 travertine mount profile represents a unique archive of the CO2-enriched fluids evolution along a single segment of a fault. The record corresponds to a period of time following the last glacial event, which locally took place 15 ky ago.

Until 6 ky

Acknowledgements

The authors would like to acknowledge Laura Crossey as well as an anonymous reviewer for their constructive comments that helped to improve this manuscript. We also would like to thanks Herman Nijs who made the thin sections, Prof. M. Joachimsky (University of Erlangen, Germany) for his support on the stable isotope measurement and IFPEN for the financial support.

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