--> Microfractures in Sandstone, by John N. Hooker, Stephen E. Laubach, Julia F. W. Gale, and Robert M. Reed, #50038 (2006).

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PSMicrofractures in Sandstone*

By

John N. Hooker1, Stephen E. Laubach1, Julia F. W. Gale1, and Robert M. Reed1

 

Search and Discovery Article #50038 (2006)

Posted October 12, 2006

 

*Adapted from poster presentation at AAPG Annual Convention, Houston, Texas, April 9-12, 2006

 

Click to view posters in PDF format, or right click to download.

Poster 1 (~0.6 mb)                  Poster 2 (~0.7 mb)

  

1The University of Texas at Austin, Austin, TX ( [email protected] )

 

Abstract 

A systematic inventory of nearly 27,000 microfractures in 64 samples of 21 sandstone units shows that fossilized (mineral-filled) microfractures are ubiquitous in rocks that have experienced moderate to deep burial (1 to 6 km) even in tectonically quiescent and otherwise structureless areas. Microfractures were detected using SEM-CL, which detects fractures despite optical continuity between host grain and fracture-filling quartz that renders fractures invisible to transmitted-light observation. Microfracture abundance is highly variable, ranging from less than 35 to more than 250 microfractures per square millimeter. Sandstones contain populations of inherited fractures, which are potentially useful provenance indicators, as well as microfractures that developed in situ. Such microfracture populations provide a record of sandstone consolidation, diagenetic history, and deformation that is not accessible using geochemical or mechanical approaches alone. Crosscutting relations and other kinematic indicators show that fracture patterns evolve with increasing consolidation, from fractures associated with grain contact in porous material to sets of subparallel fractures in solid rock. This latter fracture category commonly features isolated to interconnected pores. Some microfractures have orientation, timing, and size distribution patterns that show that they are merely smaller examples of macroscopic fractures that are present in the same rock. For these fracture types apertures range over more than three orders of magnitude (0.0001 to >0.1 mm) and commonly have size populations that can be described with power laws.

 

Selected Figures 

Color cathodoluminescence image, showing fracture sets with different color cements, La Boca Formation (Jurassic), Mexico.

Common types of microfractures in sandstone. a. Inherited fractures. b. Compactional fractures (and inherited fractures). c. Transgranular tectonic fractures (and compactional and inherited fractures). d. Microfractures in Pennsylvanian Tensleep Formation (Wyoming) that has been buried to a depth of ~5300 m.