1. CONCRETIONS in the Dakota Sandstone
By: Melissa Battler
B.Sc. Geology, University of Waterloo

2. Outline What is a Concretion? Where to Find MDRS Concretions
When were these Concretions Discovered?
Why are Concretions Significant to Mars Analog Studies?
Conclusions: How to Identify MDRS Concretions
Acknowledgements

3. What is a concretion? Short Answer:
A mass or nodule of mineral matter, usually oval or nearly spherical in shape, and occurring in sedimentary rock. It is formed by the accumulation of mineral matter in the pore spaces of the sediment, usually around a fossil or fossil fragment acting as a nucleus. Most concretions are very dense and compact, and are usually composed of calcite, silica, or iron oxide. The material making up the concretion is believed to come from the surrounding rock, being redeposited around the nucleus. Concretions range in diameter from a fraction of an inch to many feet, although most are but a few inches in diameter. Perhaps the best known are the flint nodules found in chalk deposits such as those at Dover, England. Concretions having radiating cracks filled with mineral matter are called turtle stones, or septaria.
Columbia Encyclopedia, Sixth Edition, Copyright (c) 2004:

4. Longer Answer:
A concretion is a solid mineral inclusion within a rock strata that is oval or spherical in shape. They form within layers of sedimentary strata that have already been deposited. The cementation occurs due to processes independent from the primary cementation in which the layers of sedimentary rock were adhesed together. This secondary cementation often makes the concretion harder and more resistant to weathering than the host strata.
Though the processes by which concretions form are poorly characterized, it is believed they form during the diagenesis of a deposit, usually shortly after the enclosing sediment has been buried. They are believed to occur when a considerable amount of cementing material precipitates locally around a nucleus, often organic, such as a leaf, tooth, piece of shell or fossil.
Concretions vary in shape, hardness and size, ranging from objects that require a magnifying lens to be clearly visible to huge bodies three meters in diameter and weighing several hundred pounds.
Concretions are usually similar in color to the rock in which they are found. They are commonly composed of a carbonate mineral such as calcite; an amorphous or microcrystalline form of silica such as chert, flint, or jasper; or sometimes an iron oxide or hydroxide such as goethite. They can also be composed of other sedimentary minerals that include dolomite, ankerite, siderite, pyrite, barite and gypsum, to name a few.
Concretions are found in a wide variety of rocks, and are particularly common in shales, siltstones, and sandstones. They often outwardly resemble fossils or rocks that look as if they do not belong to the strata in which they were found. Occasionally, concretions contain a fossil either as its nucleus or as a component that was incorporated during its growth, but concretions are not fossils themselves. They appear in nodular patches, concentrated along bedding planes, protruding from weathered cliffsides, randomly distributed over mudhills or perched on soft pedestals.
Because of the variety of unusual shapes, sizes and compositions, concretions have been variously interpreted to be dinosaur eggs, animal and plant fossils (called pseudofossils), extra-terrestrial debris or human artifacts. For this reason, fossil collectors commonly break open concretions in their search for fossil animal and plant specimens.
The word "concretion" is derived from the Latin "con"-- meaning "together" -- and "cresco" -- meaning "to grow."
From Wikipedia, the free encyclopedia:

5. Dakota Sandstone Concretions
The next unit – my personal favourite – is a concretion-bearing sandstone. Concretions are concentric structures that show evidence of growth outward from a nucleus. They often form around a nucleus composed of a dead organism, and grow due to bacterial decay of organic matter. The sandstone is made up mostly of 1mm and smaller quartz grains.
Typical, brown, 1cm spherical concretions

6. Dakota Sandstone Concretions
Unit 3’s large sparite crystals and isotope results suggest that the concretions developed under the water table, due to bacterial activity. This unit could also have been deposited in a beach environment.
Typical concretions on ground + brown layers that look like smeared-out concretions

7. Dakota Sandstone Concretions
Squished clusters of typical concretions + brown layers that look like smeared-out concretions

8. Dakota Sandstone Concretions
Clusters of typical brown, 1cm spherical concretions, plus loose concretions on ground

9. Petrographic Thin Section Photograph of a Typical (calcite cemented) Concretion
Iron oxy-hydroxide cement
Calcite cement
In this slide you can see a typical concretion within the host sandstone. Notice that there is poikilotopic, ferroan calcite, sparite cement inside the concretion, and empty pore space with thin micrite rims outside the concretion. The large sparite crystals inside the concretion indicate that it formed under the water table.
Concretion
Concretion
1mm
Sparite and iron-hydroxide cement inside concretion, micrite rims and pore space outside concretion, ppl & cpl

10. Iron oxy-hydroxide cement
Petrographic Thin Section Photograph of an Atypical (iron oxide cemented) Concretion
Calcite cement
Outside Concretion
Iron oxy-hydroxide cement
Inside Concretion
Micrite
This slide shows an atypical concretion, which formed differently from the last concretion. It features zoning (outlined in yellow), micrite rims, and iron-hydroxide cement inside the concretion, and poikilotopic sparite cement outside of the concretion. The concretion is very dirty-looking, and therefore might contain a lot of organic material.
Zoning in a concretion with micrite rims and iron-hydroxide cement
zoning

11. Where to Find Concretions
Concretions can be found in the Cretaceous Dakota Sandstone Formation around MDRS, near Hanksville, Utah
The Dakota Sandstone Formation is locally a 1 to 20m thick unit of yellow and brownish sandstones and conglomerates, capping the shaley, rainbow coloured hills of the Jurassic Morrison Formation
The Dakota Sst can be further subdivided into 6 sub-units; Unit 3 contains concretions
There are several known concretion sites around MDRS, however the full extent has not yet been mapped

12. Geological Map of the Hanksville area, showing general extent of the Dakota Sandstone Formation
Mancos Shale J1 K2 J2
Cretaceous K1
Dakota Sandstone
MDRS
Morrison Formation J2
Hanksville K1
Jurassic
Entrada Sandstone K2 J1
10km N
Stratigraphic cross-section showing position of Dakota relative to Morrison

13. Dakota Sandstone Morrison Formation
First, starting with the base of the Dakota Sandstone, is the Green Conglomerate. This unit featured clasts of very poorly sorted, rounded, fine-to-medium sized chert pebbles, a matrix of well sorted, subrounded, very fine quartz granules, and 3 stages of cementation.

14. Units of the Dakota Sandstone Formation
Oyster Conglomerate & Sandstone
Burrowed Sandstone
Coal & Shale
15 m
Sandstone with Concretions
Intertonguing Conglomerate & Sandstone
I’ve outlined the 5 units that we can see here; at the base, not seen here is a Green Conglomerate. Overlaying that conglomerate is a unit of Intertonguing Conglomerate and Sandstone, overlain by a unit of Sandstone with Concretions, overlain by a unit of Coal and Shale, overlain by a Burrowed Sandstone, and finally, overlain by a unit of fossiliferous conglomerate and sandstone.
Basal Green Conglomerate unit not seen here
Units of the Dakota Sandstone Formation

15. Composite Cross Section
Present erosional surface
(Mancos Shale lies conformably above; not seen during this study)
Composite Cross Section
Unit #6: Oyster Conglomerate and Sandstone
Unit #5: Burrowed Sandstone
Unit #4: Coal and Shale
15m
Unit #3: Sandstone with Concretions
This is a composite cross section I’ve created of all 6 of the units. The pink dots to the left of the column show where all my samples were taken from, and the blue arrows show which samples were made into thin sections. I will now go through each unit, one at a time, showing the results of my petrographic analysis.
Unit #2: Intertonguing Conglomerate and Sandstone
LEGEND
Coal
Oyster fossils
Sandstone
Burrows
Cross- bedding
Unit #1: Green Conglomerate
Sample Location
Clasts
Shale
Concretions
Thin Section
Morrison Formation below unconformity
Conglomerate

16. When were these concretions discovered?
Melissa Battler & Rocky Persaud first found the MDRS concretions during Expedition One, in February 2003
Concretions have been documented in other parts of the Dakota Sandstone, such as the 1-2m concretions found in Kansas over 100 years ago
Rocky, Melissa & Matt
Dakota
Sandstone
UTAH
The Dakota Sandstone is a very widespread unit, covering many parts of central USA
Kansas concretions

17. Why are concretions significant to Mars analog studies?
Because concretions are often formed biogenically, and therefore indirectly indicate the past presence of life
Concretions require water to form, and therefore directly imply the past presence of water
Concretions have been discovered on Mars!!!!!

18. Mars Exploration Rover Data

19. Concretions on Mars!
Opportunity found spherules that appear to be concretions at the Meridiani Planum site!
Solid homogeneous spheres with no banding or weathering rings
Colour difference between concretions and host rock
Weather out in spheres
Cluster of 3 spheres had to grow radially outwards & then join together, to form this shape. This gives strong evidence that these are concretions of sedimentary origin, and not pizzoliths of volcanic origin
NASA’s Opportunity rover has found what appear to be concretions on Mars, at the Meridiani Planum site! Like the Dakota Sandstone concretions, the Martian concretions are Solid homogeneous spheres with no banding or weathering rings, there is a Colour difference between concretions and host rock, and the concretions Weather out in spheres! There are alternative explanations for these structures, including volcanic origin, however If the spherules had not been concretions, the rock layers above and below them would have appeared deformed. Therefore it seems that most of the Mars Exploration Rover geologists believe that these are concretions.

20. Martian Outcrop where Concretions were Discovered

21. More Pictures of Martian Concretions

22. Martian vs. Earth ConcretionsB
To compare, here is a picture of a Martian concretion on the left, and some Dakota concretions on the right. Most concretions on Earth are formed due to bacterial activity, so it is exciting to note that these concretions look strikingly similar to those recently identified on Mars!
1cm
1cm
Mars
MDRS

23. It is clear that the Mars & MDRS Dakota Sandstone concretions look strikingly similar. Isotopic data from the MDRS concretions suggest that these were formed biogenically… perhaps the Mars concretions were too…? We need significantly more data to answer this question. A B
To compare, here is a picture of a Martian concretion on the left, and some Dakota concretions on the right. Most concretions on Earth are formed due to bacterial activity, so it is exciting to note that these concretions look strikingly similar to those recently identified on Mars!
1cm
1cm
Mars
MDRS

24. Conclusions: How to Find Concretions around MDRS
Look in yellow, fine-grained sandstone unit of the Dakota Sandstone Formation (Unit 3)
For a more complete description of Unit 3, refer to
Look for weathering-resistant spherical balls of sandstone, roughly 1cm in diameter
I.e. harder than the surrounding host-rock
Might be oval-shaped, or appear to be smeared into layers
Can be brown, black, white, or orange
Note: these are just guidelines. It is possible that atypical concretions exist within the Dakota Sandstone, and it is also possible that concretions may exist within other geological units around MDRS

25. Resources
Paleoenvironmental Interpretation of the Dakota Sandstone as a Mars Analog, focusing on Concretions
Columbia Encyclopedia, Sixth Edition, Copyright (c) 2004:
From Wikipedia, the free encyclopedia:
Hintze, L.F Geologic History of Utah: A Field Guide to Utah’s Rocks. Edited by B.J.Kowallis. Brigham Young University Geology Studies, Special Publication 7, 202pp.
Lawyer, G.F Sedimentary Features and Paleoenvironment of the Dakota Sandstone (Early Upper Cretaceous) Near Hanksville, Utah. Brigham Young University Research Studies, Geology Series, 19:
Mozley, P.S The Internal Structure of Carbonate Concretions in Mudrocks: a Critical Evaluation of the Conventional Concentric Model of Concretion Growth. Sedimentary Geology, 103:
Shaffer, H.L Concretions in the Dakota Sandstone. The Compass of Sigma Gamma Epsilon, 17:
Stamm, N USGS National Geologic Map Database - Geologic Unit Name: Dakota.
Webster, G. and Brown, D Mineral in Mars 'Berries' Adds to Water Story. NASA News Release: , March 18,
Finally, I would like to thank all these people for all of their help and excellent advice along the way. Thank you.
Good things to know:
-Chalcedony often occurs as a replacement cement in carbonates, and is a cement in chert
-Calcrete (Caliche) is originally calcite and replaced by silica