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Description:
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Present models suggest that fluvial and marine depositional patterns were distinct
from modern patterns prior to the appearance of land plants . Although these models are
likely correct , problems exist when one attempts to distinguish between fluvial and
shallow marine deposits in pre -Silurian strata , making it difficult to accurately determine
depositional patterns . The lack of land plants and scarcity of body and trace fossils ,
especially in Precambrian and early Cambrian strata , make identification difficult .
Based on core data and limited outcrops , the Lower Hickory Sandstone , a late Cambrian
sandstone , has been interpreted to progress from fluvial to shallow marine . These data
have allowed the development of an overall depositional model , but minimal detail of
facies changes is available . Based on the limited data , both deltaic and estuarine models
have been suggested for the Lower Hickory .
Mining of the Lower Hickory for frac sand has created highwalls in the
CarmeuseNA Mine , which provides an opportunity to study facies changes at this site .
The CarmeuseNA Mine , located in McCulloch County , Texas , has exposed the
formation along ~500 m long and 20 m -high faces , respectively . Because of limited exposure , only the south and west walls , as well as part of the east wall , could be
examined . Digital photographs of the faces were mosaiced using standard
photogrammetrical practices to produce visual representation of the highwalls . Bedding
geometry was then mapped on the digital images to facilitate a detailed interpretation of
the depositional process . Core and well data were used to map Hickory thickness to
produce an isopach map .
Four primary facies were observed in the quarry , dominated by small -scale and
large -scale cross -bedding . Paleocurrents are generally unidirectional to the southsoutheast
indicating a braided fluvial origin , but rarely opposing directions are seen .
Bioturbation is rare low in the section , but increases upwards . Together with the rare
herringbone cross -bedding , clay drapes , and bioturbation , a tidal influence is strongly
suggested . The model suggested is a braided stream setting influenced and reworked by
tides . A braided -delta fed by braided streams guided by a ridge and swale -dominated
setting , which served as the sediment supply for the delta , is proposed . |