VIII. AcknowledgmentsSpecial thanks to Trinity University and the Geoscience Department for their funding and logistical support, with-out which this research would not have been possible. I would also like to thank the Black Gap Wildlife Manage-ment Area and especially Mike Pittman and Travis Smith for logistical support. Thanks also to Luciana de la Rocha for help in the field and in the bunkhouse.

The Relationship between Topographic Profile and Mechanical Stratigraphy in the Stillwell Anticline, West TexasLauren Mercado, Ben Surpless, Department of Geoscience, Trinity University, San Antonio, TX 78212

VII. DiscussionI found that the mechanical strength of individual layers is not correlated with the slope; instead, other factors such as bed thickness and the presence or ab-sence of interlayer slip apparently play an important role in controlling topo-graphic profile and possibly the mechanical stratigraphy for a given rock forma-tion.

The joint data shows that there are one or two distinct sets in each unit of the an-ticline that vary in orientation from one unit to the next and do not relate to the more uniform orientation of dominate regional joint sets, suggesting that stresses deviated locally and that each unit behaves independently of its neigh-bors with little coupling between these units. I also observed that joints tend to terminate at bedding planes because weak interfaces reduce the stress at the tip of the joint, not allowing it to go across the bed plane (e.g., Erdogan, 1972; Rijken and Cooke, 2001).

These results have impolications for future research investigating fracture propagation during fold formation, since it is apparent that fractures are strongly affected by bedding planes. The mechanical decoupling at layer inter-faces potentially changes fluid flow paths in subsurface fold systems., so must be considered in developing hydrologic and petroleum hydrologic models.

Uni

t I (1

4.37

m)

Uni

t II (

6.81

m)

Uni

t III

(9.9

0m)

Uni

t IV

(5.8

7m)

Uni

t V(2

.50m

)U

nit V

I (3.

43m

)U

nit V

II (9

.13m

)U

nit V

III (6

.86m

)

Units Meters above base

Graphic Columnar section

10

20

30

40

50

30

Strengthsin Q

40 80706050 90

Average= 62.3

Average= 59.8

Average= 62.8

Average= 51.1

Average= 59.2

Average= 59.1

Average= 62.4

Average= 61.9

Unit Description

Limestone, weathers to light grey, no fossils, thickness shown is minumum (no upper contact exposed). Lower section of unit consists of beds ranging in thickness from 15 to 61 cm. Middle section consists of beds 1.8 and 2.62 m thick. The upper most bed is 40 cm thick.

Limestone, weathers to light grey, chert fossils dispersed throughout unit. Beds range in thickness from .74 to 2.55 m.

Limestone, weathers to light grey, no fossils, shallower sloped. Beds range in thick-ness form 48 to 93cm.

Limestone, no fossils, one layer 2.50 m thick, cliff form-ing.

Limestone, weathers to light grey, chert bivalve fossils found in the lower three layers. Beds range in thick-ness from .42 to 2.34 m.

Limestone and chert, lime-stone weathers to light grey. Upper section consits of beds 1.12 and 3.60 m thick contain-ing latterally discontinious chert nodules and chert fossils. Lower section consists of interbedded limestone and chert ranging in thickness from 2 to 48cm.

Limestone, weathers to light grey, no fossils. Upper layers contain chert nodules throughout, but lower layers do not contain chert nodules. Beds range in thickness from .58 to 2.70 m.

Limestone, weathers to light grey, fossils in lowest layer. Lowest layer is 3.80 m thick. Middle section in 7.4 m thick and was not exposed at the surface. Upper section ranges in thickness from .51 to 1.68 m.

n= 20

N

n= 20

N

n= 20

N

n= 20

N

n= 25

N

Joints (Lower Hem. and Equal Area)

n= 25

N

n= 1

N

Unit VIII

Unit VII

Unit VI

Unit V

Unit IV

Unit II

Unit I

Unit Ia

Unit Ia

Unit Ib

Unit IIb

Unit IIb

Unit IVa

Unit IVa

Unit IVb

Unit IVb

Unit VaUnit Vb

Unit VIa

Unit VIa

Unit VIIa

Unit VIIb

Unit VIIIa

VI. Results: Stratigraphic Data

Very few joints on the Stillwell anticline propagated from one bed to the next, and I documented joint sets with different orientations in each unit, suggesting very localized stress fields. In addition, these units display joint sets that are inconsistent with the dominant, northwest-trending regional joint set (see section V.), also indicating local stress field pertur-bations. These differences could be related to a combination of joint for-mation during different stages of fold formation, and likely suggests de-coupling of these units during deformation.

Q-value, obrtained with Schmidt hammer, is a measure of compressive strength. Q values displayed above suggest that simple compressive strength cannot be related to weathering profile or slope (see Graphic co-lumnar section, above, and photo shown in section V). The Q-values remain relatively constant throughout the stratagraphic column in spite of signifi-cant changes in slope and/or weathering profile. This strongly suggests that other factors, including layer thickness and/or interlayer coupling, influence slope and, potentially, mechanical stratigraphy.

Erdogan, F., 1972, Fracture problems in composite materials: Journal of Engineering and Fracture Mechanics, v. 4, p. 811 – 840.Laubach, S., Olson, J., and Gross, M., 2009, Mechanical and fracture stratigraphy: American Association of Petroleum Geolo gists Bulletin, v. 93, p. 1413-1426Lehman, T., and Busbey, A., 2007, Society of Vertebrate Paleontology Fall 2007 Big Bend field trip guide: Austin, Texas, Society of Vertebrate Paleontology, 117 p.Miller, D., Milsen, T., and Bilodeau, W., 1992, Late Cretaceous to early Eocene geologic evolution of the U.S. Cordillera: In Burch fiel, B., Lipman, P., and Zoback, M., Eds., The Cordilleran Orogen: conterminous U.S.: Geologic Society of America, v. G-3, p. 2005-260Muehlberger, W.R., 1980, Texas Lineament Revisited: New Mexico Geological Society Guidebook, 31st Field Conference, Trans-Pecos Region, p. 113 – 121.Muehlberger, W.R., and Dickerson, P.W., 1989, A tectonic history of Trans-Pecos Texas, In Muehlberger, W.R., and Dickerson, P .W., Eds., Structure and stratigraphy of Trans- Pecos Texas: American Geophysical Union Field Trip Guidebook T315, p. 35-54Page, W., Turner, K., and Bohannon, R., 2008, Geological, Geochemical, and Geophysical Studies by the U.S. Geological Survey in Big Bend National Park, Texas, In Gray, J., and Page, W., Eds., U.S. Geological Survey Circular 1327, 93 p.Rijken, P., and Cooke, M., 2001, Role of shale thickness on vertical connectivity of fractures: application of crack-bridging theory to the Austin Chalk, Texas: Tectonophysics, v. 337, p. 117 – 133.Shackleton, J., Cooke, M., and Sussman, A., 2005, Evidence for temporally changing mechanical stratigraphy and effects on joint-network architecture: Geology, v. 33, p. 101 – 104.Smart, K., Ferrill, D., and Morris, A., 2009, Impact of interlayer slip on fracture prediction from geomechanical models of fault- related folds: American Association of Petroleum Geologists Bulletin, v. 93, p. 1447-1458St. John, B.E., 1965, Structural geology of Black Gap area, Brewster Country, Texas: Ph.D. Thesis, University of Texas at Austin, Austin, Texas, 200 p.

V. Results

Slope-defined Mechanical Units Unit I

Unit II

Unit III

Unit IV

Unit VUnit VI

Unit VII

Unit VIII

300m

0m

50m

100m

150m

200m

250m

Sant

a El

ena

Form

atio

n

Regional Joint Sets

On the regional scale, adjacent to the anticline, there are two primary joint sets (see figure below). The dominant northwest-trending joint set to the northeast (yellow lines in figure below) has an average spacing of 55.0 meters with a standard deviation of 10.3 meters. The less prominent northeast trending joint set (pink lines in figure below) has a much wider spacing and thus a lower density. These joint sets are likely related to far-field stresses that affected much of the Big Bend area. I can use these joints to determine the relationship between regional and local defor-mation.

I defined possible mechanical units based on changes in slope of the Santa Elena limestone, where the limestone is best exposed in the anticline (see black boxs shown in “Methods” section figure for position of the photo shown to the right). My initial hypothesis is that rock strength can be directly related to slope, with stronger units displaying steeper slopes than weaker units. If true, then we can use this topographic profile to estimate mechanical stratigraphic units. The po-sition of Units I through VIII is shown relative to the entire Santa Elena limestone stratigraphic column (see figures, right). The colors and units shown here correlate with the Stratigraphic data shown above and to the right.

IV. MethodsTo investigate the mechanical stratigraphy of the Stillwell anticline, I took compressive strength and joint measurements of each layer within the central section of the anticline system (see figure, right). I first mea-sured the bed thickness of all the beds in the middle of the anticline using a Jacob staff. I then grouped the beds into eight units based on slope differences and created a topographic profile to illustrate these differ-ences. After creating the profile I measured the strength of each bed within each unit with the Schmidt hammer, which uses impact testing to estimate compressive strength of materials. To get accurate data with the Schmidt hammer, I measured only relatively flat, minimally-weathered surfaces away from frac-tures. I took ten closely-spaced measurements and found the mean and standard deviation of these mea-surements. I also used the Brunton compass to measure the orientation of joints in each of the eight me-chanical units, and I used a measuring tape to obtain the spacing joints.

SSSSSSSSSSSSttttttttttiiiiiiiiiiiillllllllllllllllllllllllwwwwwwwwwwwwweeeeeeeeeeeeelllllllllllllllllllllllll AAAAAAAAAAAnnnnnnnnnnnttttttttttttiiiiiiiiiiiiccccccccccccclllllllllllliiiiiiiiiinnnnnnnnnnnnneeeeeeeeeee

NN

500 m

III. Research BackgroundMechanical StratigraphyMechanical stratigraphy is based on rock properties such as tensile strength, elastic stiffness, and other rock properties that determine how a given layer or layers will deform due to applied stresses. Deformation is con-trolled in part distribution within a stratagraphic column of the rigidity of each unit, the relative thicknesses of individual layers, and the nature of the interfaces between those layers (e.g., Cook and Erdogan, 1972; La-deira and Price, 1981; Underwood et al., 2003). Mechanical stratigraphy also influences joint propagation and orientation. The differences in mechanical stratigraphy post deposition can lead to different joint patterns even though variables such as thickness and lithology can remain constant (Shackleton et al., 2005). Jointing can occur any time after deposition, and when joint sets form before or during folding events, the sets can rotate so they no longer align with previous joint sets. When mechanical stratigraphy remains relatively constant, ranges in fracture intensity can occur because of different amounts and types of deformation at different locations (Laubach et al., 2009).

Stress and Joint Evolution Stresses can be applied on two different scales. Far-field stress acts on a wide region and is caused by large scale tectonic processes, such as the Laramide Orogeny, which causes the fractures to have consistent orientations. Local perturbations of stresses may cause fractures to be oriented in different directions or have different magnitudes. Joints are planar fractures that cut through rocks leaving little or no displacement. Part of my research involves joint formation, propagation, and termination. Joints form parallel to the axis of bending and perpendicular to the stretching. As stress contin-ues, the pre-existing joints become longer and new joints form. However, joints do not nucleate if they are within a certain distance from a pre-existing joint, controlled by variables such as bed thickness, mechanical properties, loading history, and total strain. There are several ways in which joints terminate. When two joints get too close, shear stresses make the joints terminate.

tttrir nityuuttnniversitityyttttttttrir nityrinityyrir nityrir nity

uuuutt

uuuuuuutttttttt

ininniversityyniniversitityyninin versitittyyy

I. IntroductionMechanical stratigraphy is defined by grouping layers of rock with similar ten-sile strength, elastic stiffness, brittleness and fracture properties. Researchers can use a developed mechanical stratigraphy to predict how layers of rock will deform through time, such as when those layers are folded during compres-sion. In addition, it has been hypothesized that a rock-slope will naturally reach equilibrium based on its strength, so I investigated the relationship be-tween slope-defined mechanical stratigraphic units (using topographic pro-file) and compressive strength, using joint orientations to help determine po-tential coupling between units during compression-related deformation. To do this, I studied the well-defined Stillwell anticline, a Laramide-age fold lo-cated east of Big Bend National Park.

I focused on the following research questions:

II. Tectonic Setting

NE SW50 m

Shaded relief map with major Laramide-age faults and folds of the Big Bend region (in red), with inset (lower right) showing distribu on of deforma on associated with the Laramide orogeny (red shading, inset) and the approximate boundaries of the Texas Lineament (TL, inset). The Sierra del Carmen mountains (SDC) and the San ago moun-tains (SM) are shaded in yellow. Big Bend Na onal Park (BBNP) is outlined green and the S llwell an cline is labeled SA. Distribu on of Laramide-age faults and folds modified from Muehlberger and Dickerson (1989). Laramide orogeny modified from Miller et al. (1992). Approximate boundaries of the Texas Lineament modified from Muehlberger (1980). Figure modified from Surpless and Quiroz (2010).

BBNP

Mexico

USA

SA

fold:

fault ormonocline:

anticline:

Marathon uplift(Pz strata exposed)

N

0 25

km

KEY:

SM

SDC

TX

Mexico

PacificOcean

N

500 km

Laramid

eo

rog

eny

TLTL

The northeast-trending Texas lineament is defined by a series of subparallel faults, folds, and fractures in west Texas and northern Mexico (see figure, right). These structures are thought to be controlled by earlier tectonic events which first affected the area during Neoproterozoic rifting (e.g., Page et al., 2008). During the Triassic age, rifting between the North and South American plates began (e.g., Muehl-berger and Dickerson, 1989), causing subsidence in the study area, permitting shallow marine carbonate and shale deposition. The Cretaceous Santa Elena limestone, which is best exposed in the Stillwell anticline, was formed during this period.

Increased rates of subduction of the Pacific plate under the North American plate ended subsidence and marine deposi-tion and led to the compression-related faults and folds of the Laramide Orogeny, with features that extend through the study area (see figure, right; e.g., Muehlberger and Dick-erson, 1989). The Stillwell anticline (SA in figure, right) was formed during this period of compression, which came to an end in late Eocene or Oligocene time (e.g., Page et al., 2008).

1. How does the compressive strength of layers correlate with slope?

2. How do joint orientation and spacing vary within and between stratigraphic units and what factors contribute to this variation?

3. How much mechanical coupling occurs between mechanical strati- graphic units during deformation?

IX. References