Blakely Sandstone

{{Infobox rockunit

| name = Blakely Sandstone

| image = Quartzose sandstone (Blakely Sandstone, Middle Ordovician; Coleman Quartz Mine, Arkansas, USA) (8290507503).jpg

| caption = Blakely Sandstone (Coleman Quartz Mine, Arkansas)

| type = Formation

| age = Ordovician

| prilithology = Sandstone

| otherlithology =

| namedfor = Blakely Mountain, Garland County, Arkansas

| namedby = Albert Homer Purdue{{cite journal|last1=Ulrich|first1=E.O.|title=Revision of the Paleozoic systems|journal=Bulletin of the Geological Society of America|date=1911|volume=22|issue=1|page=677|doi=10.1130/GSAB-22-281|bibcode=1911GSAB...22..281U|doi-access=free}}

| region = Arkansas, Oklahoma

| country = United States

| coordinates =

| unitof = none

| subunits = none

| underlies = Womble Shale

| overlies = Mazarn Shale

| thickness = up to 700 feet{{cite journal|last1=McFarland|first1=John David|title=Stratigraphic summary of Arkansas|journal=Arkansas Geological Commission Information Circular|date=2004|orig-year=1998|volume=36|page=19|url=http://geology.ar.gov/pdf/IC-36_v.pdf|access-date=2018-01-08|archive-url=https://web.archive.org/web/20161221195953/http://www.geology.ar.gov/pdf/IC-36_v.pdf|archive-date=2016-12-21|url-status=dead}}

| extent =

| area =

| map =

| map_caption =

}}

The Blakely Sandstone is a Middle Ordovician geologic formation in the Ouachita Mountains of Arkansas and Oklahoma. First described in 1892,{{cite journal|last1=Griswold|first1=L.S.|title=Whetstones and the novaculites|journal=Annual Report of the Geological Survey of Arkansas for 1890|date=1892|volume=3}} this unit was not named until 1909 by Albert Homer Purdue in his study of the Ouachita Mountains of Arkansas. Purdue had initially named this unit the Caddo Shale at a 1907 Geological Society of America meeting,{{cite journal|last1=Purdue|first1=A.H.|title=Structure and stratigraphy of the Ouachita Ordovician area, Arkansas (abstract)|journal=Geological Society of America Bulletin|date=1909|volume=19|pages=557|doi=10.1130/GSAB-19-513|url=https://zenodo.org/record/1716453}} but later redefined and renamed the unit as the Ouachita Shale.{{cite book|last1=Purdue|first1=A.H.|title=Slates of Arkansas|date=1909|publisher=Geological Survey of Arkansas}} He again renamed the unit to the Blakely Sandstone in a letter to Edward Oscar Ulrich, to which Ulrich used in a 1911 publication, becoming the first reference using this name. Ulrich assigned the Blakely Mountain in Garland County, Arkansas as the type locality, but did not designate a stratotype. As of 2017, a reference section for this unit has yet to be designated.