Clastic dikes in the Tertiary strata of Badlands NP

Harmon D. Maher Jr. 7/2009, University of Nebraska at Omaha, Dept. of Geography and Geology

Introduction

The following is background information provided for a undergraduate research project on the origin of chalcedony veins in Tertiary strata of Nebraska and South Dakota.

Clastic dikes are cross-cutting (discordant), tabular bodies that are filled with sediment. That cracks that fill with magma cause igneous dikes is well known. However, sediment can also fill a crack. There has been and is debate on how they form. A polygenetic origin may be contributing to this debate. Two end members models for how they form are: 1) that the cracks were open to the surface and fill in with sediment from above, or 2) they form as buried sediment is liquefied and injects itself into cracks. One might call the first model fissure fill, and the second model injection. Some people have taken to calling clastic dikes as a type of injectite (good key word to search the literture with). Injectites are often considered to involve upwards movement from a source bed, but horizontal and/or downward movement is quite possible. One possible idea for their genesis is described in this GSA abstract.

Clastic dikes are common in the Tertiary strata we will be working on. In addition, two other aspects may them of interest in our research. First, in places they are parallel to and 'intruded' by chalcedony veins, and/or form composite dike-vein bodies. There may be a broad spatial association between the two. This suggests several possible connections between the chalcedony veins and the clastic dikes. Second, recent literature suggests that some injectites may also be fueled by structural diagenesis (Davies abstract), so the clastic dikes may (and I stress may) be part of a larger picture. A general question then is what are the differences and similarities between the two. Below are some images with comments to further introduce you to these fascinating features.

In this photo of an area along the main paved road through the North Unit of the NP, one sees an array of subvertical lines cutting through the Tertiary strata. These are clastic dikes. Note the multiple directions they take, and one question we can ask, is - do they have a preferred orientation, and if so what is it. Note also the the dikes clearly cross cut each other. The strata seen are the upper part of the Brule Fm., with the Sharps Fm. not to much above this level.

This image clearly shows that the dike fill is not uniform. In its internal features is information about the processes that played a role in their formation. The red coloration is a mixture of red and green muds. Simplistically, red muds are associated with oxidizing fluids and green with reducing. It appears there has been some mixing and/or juxatposition of sediments from different sources and out of equilibrium with each other. Another possibility is movement of fluids along the dike along fractures or paths that produced alteration of the original mud.

In this image more complexities canm be seen. Because of the similarities with the surround sediment it is a bit harder to see what is dike material and what is country rock. However, after you train your eye, you can see that this is a composite dike, with a light tan mud fill, a dark brownish mud fill and a light tan sand fill. In addition in the sand fill there are what appear to be cross-beds! One can argue that this dike opened up 4 different times with a different sediment fill each time.

The clastic dike here is much thinner, and is cutting more massive siltstone of the Brule Formation. Note how as you trace it that in cross section it consists of two segements with overlapping tips. Elsewhere bends and branches are seen. The three dimenional geometry of these may also provide insight into how the dikes formed.

This is a badlands cliff side and so is a subvertical view of a light green siltstone dike cutting the brown siltsone (Brule Fm. again). Of note here is the thin lense of country rock embeded within the dike. This might be called a septa. What does it suggest about the mechanics of forming this dike. The septa seems to be the same material as the adjacent layer, and does not seem to have traveled far if at all. Note also the beautiful popcorn texture developed in the country rock material.

This is another subvertical view of a relatively thin sandstone dike. Note the very clear V shaped laminae in the sand that point upwards. The easiest way to interpret this is as upwards fluid flow rearranging sand grains. Remember that upwards vertical flow could be the last fluid flow movement recorded and not necessarily that associated with initial sediment emplacement.

This is a close up of another dike. The dike material here has undergone cementation (carbonate?) and the softer country rock material has eroded away resulting in exposure of vertical 'flute' marks. These slant inwards and upwards, again consistent with upwards fluid flow. These features are not uncommon in the dikes.

One can consider what traits you would expect the dikes to have if they are due to structural diagenesis, so that you can test this hypothesis.