View source for Shadow Lake Formation

{{Infobox rockunit
| name           = Shadow Lake formation
| image          =
| caption        =
| type           = [[Geological formation]]
| age            = [[Middle Ordovician]]
| period         = Middle Ordovician
| prilithology   = [[Sandstone]], [[conglomerate (geology)|conglomerate]]
| otherlithology = [[Siltstone]], [[shale]]
| namedfor       = [[Shadow Lake (Kawartha Lakes)|Shadow Lake]]
| namedby        = Liberty
| year_ts        = 1969
| region         = [[Ontario]]<br />[[Pennsylvania]]
| country        = {{CAN}}<br />{{USA}}
| coordinates    =
| unitof         = [[Black River Group|Black River]] & [[Simcoe Group]]s
| subunits       =
| underlies      = [[Gull River Formation]]
| overlies       = [[Grenville orogeny|Grenville basement]]<br />[[Loysburg Formation]]
| thickness      = {{convert|0|-|10|m|ft|abbr=on}}<ref name=OGS_p8/>
| extent         = 
| area           =
| map            =
| map_caption    =
}}
The '''Shadow Lake formation''' is a [[geological unit]] that crops out in [[Southern Ontario]], [[Canada]] and northern [[Pennsylvania]], [[United States]].<ref name="ont">{{cite web | url=http://www.mndm.gov.on.ca/mines/ogs/resgeol/geology/sw_e.asp | title=Geology of the Southwestern Ontario District | publisher=Ontario Ministry of Northern Development and Mines | date=25 November 2011 | accessdate=14 April 2012}}</ref> The [[shale|shaly]] sections act as a [[caprock]] to [[petroleum reservoir]]s.

== Description ==
The oldest [[Paleozoic]] strata in the north Lake Simcoe area consist of the mainly siliciclastic sediments of the Shadow Lake Formation. These strata lie unconformably and sharply on the Precambrian Grenville basement, however, this basal contact is rarely exposed. The Shadow Lake Formation is poorly exposed, typically occurring in small isolated outcrops along a low escarpment capped by the more resistant carbonate beds of the overlying Gull River Formation. Trending roughly east—west across the study area and ringing Paleozoic outliers, this escarpment marks the edge of the Paleozoic outcrop belt.<ref name=OGS_p8>Armstrong, 2000, p.8</ref>

The Shadow Lake Formation consists of non-fossiliferous, red, maroon and green, poorly sorted, [[argillaceous]], arkosic [[sandstone]]s and [[conglomerate (geology)|conglomerates]], arenaceous (sandy) siliciclastic [[mudstone]]s ([[shale]]s) and [[siltstone]]s, and minor argillaceous [[Dolomite (rock)|dolomite]] and [[limestone]]s. These rock types are commonly interbedded and gradational with each other.<ref name=OGS_p8/>

The Shadow Lake Formation, originally named by Okulitch (1939) based on outcrops north of [[Coboconk]], was redefined by Liberty (1969). Okulitch included dolomitic limestones in the top of Shadow Lake Formation, whereas Liberty assigned these strata to the overlying [[Gull River Formation]].<ref name=OGS_p8/>

The siliciclastic sediments of the Shadow Lake Formation are gradationally overlain by impure carbonates of the Gull River Formation. The upper contact of the Shadow Lake Formation is placed where its mainly siliciclastic sediments give way to mainly carbonate sediments (typically argillaceous dolomites) of the lower Gull River Formation. Natural weathering accentuates this contact, making it appear sharp in outcrop, whereas in drill core this contact can be difficult to pick out.<ref name=OGS_p8/>

In Ontario, the Shadow Lake Formation ranges from {{convert|0|to|10|m|ft}} in thickness, and averages approximately {{convert|6|m|ft}} in thickness.<ref name=OGS_p8/>

The formation also crops out in Pennsylvania, where it is unconformably overlying the [[Loysburg Formation]].<ref name=USGS>[https://pubs.usgs.gov/pp/1708/e21/pdf/i-2200.pdf 1992 - Ryder - Stratigraphic framework of Cambrian and Ordovician rocks in the Central Appalachian Basin from Lake County, Ohio to Juniata County, Pennsylvania] - [[USGS]]</ref>

== References ==
{{Portal|Geology}}
{{reflist}}

=== Bibliography ===
* {{citation |last=Armstrong |first=D.K |year=2000 |title=Paleozoic Geology of the Northern Lake Simcoe Area, South-Central Ontario |url=http://www.geologyontario.mndmf.gov.on.ca/mndmfiles/pub/data/imaging/OFR6011/OFR6011.pdf |publisher=Ontario Geological Survey |pages=1–47 |accessdate=2018-08-07}}

[[Category:Geologic formations of Ontario]]
[[Category:Geologic formations of the United States]]
[[Category:Ordovician Ontario]]
[[Category:Ordovician United States]]
[[Category:Sandstone formations of Canada]]
[[Category:Shale formations]]
[[Category:Sandstone formations of the United States]]
[[Category:Shale formations of the United States]]
[[Category:Conglomerate formations]]
[[Category:Siltstone formations]]
[[Category:Seal rock formations]]