File:Sandstone injectite with granite pebbles (Tava Sandstone, Neoproterozoic; west of Woodland Park, Colorado, USA) 1.jpg

English: Sandstone injectite with granite pebbles from the Precambrian of Colorado, USA. (cut surface)

Dikes are planar igneous intrusions that cut across country rocks. (See: www.flickr.com/photos/jsjgeology/albums/72157632467411234) Common lithologies in igneous dikes include granite and basalt/diabase. Some dikes are composed of sedimentary material - such clastic dikes form when material fills in fractures in other rocks. (See: www.flickr.com/photos/jsjgeology/albums/72157711855111128)

The rare rock seen here is from a sandstone injectite dike. It is composed of lithified sand with granules and pebbles. The dike is hosted by coarsely-crystalline granites of the Pikes Peak Granite. Angular pieces of non-sheared, wall rock granite are in the dike. The dike fill is ~90% quartz sand and ~10% potassium feldspar sand. Mudstone pebbles have been reported from some examples. The clastic dikes in this part of Colorado have been known since the 1890s - gold explorers examined them. For over 100 years, the sandstone dikes have been interpreted as injectites (forcefully injected sand intrusives) - these are some of the largest on Earth. Major and minor sandstone dikes occur - some are over 100 meters across. About 200 dikes have been mapped, plus questionable examples in Boulder County. The dikes are well-cemented and solid in places and crumbly at other sites. Some compound injections are known, representing multiple injection events.

The origin of Colorado's sandstone injectite dikes has long been a mystery. The basic idea of overpressured, non-lithified, sandy material being forcefully injected downward into fractured granite is generally accepted, but the timing and the exact source of the material is traditionally undetermined and controversial. Many researchers concluded that the Cambrian-aged Sawatch Sandstone was the source of the sand, because it is the basal Phanerozoic unit atop this area's Precambrian crystalline basement rocks. Did the Sawatch get remobilized during the Laramide Orogeny (= late Mesozoic to early Cenozoic mountain-building event that formed the true Rocky Mountains)? That's a strange idea - it's implausible that a Cambrian sandstone would stay nonlithified for over one-third of a billion years. Some think that injection occurred during the Ancestral Rockies Uplift (Late Paleozoic), or the Sawatch got remobilized in the Early Paleozoic. Regardless of timing, how can a ~5 meter thick Sawatch Sandstone deposit be the source for 100 meter thick sandstone dikes? Maybe the Sawatch was originally thicker, but is now mostly eroded away. Another interpretation is that these are thin and thick fault slices.

Detrital zircon analysis of sandstone injectite dikes in central Colorado has shown that the original source of sandy material is a long eroded-away, late Precambrian-aged unit that's been designated the Tava Sandstone.

Geologic unit: injectite dike of Tava Sandstone, Neoproterozoic, possibly ~680 to 800 Ma

Locality: roadcut along access road in Majestic Park gated neighborhood, west of the town of Woodland Park, northeastern Teller County, central Colorado, USA (39° 00’ 04.04” North latitude, 105° 05’ 18.54” West longitude)

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Some info. from:

Siddoway & Gehrels (2014) - Basement-hosted sandstone injectites of Colorado: a vestige of the Neoproterozoic revealed through detrital zircon provenance analysis. Lithosphere 6: 403-408.