Series: USGS Miscellaneous Investigations Series 1-919 



Date: 1976 



Detailed regional geologic mapping at 1:125,000 scale shows the 

 sample site and most of Halfway Lake underlain by the Judith River 

 Formation of late Cretaceous age. The northernmost arm of the lake 

 extends across a minor normal fault and into a small graben 

 preserving late Cretaceous Bearpaw Shale and a small area of 

 Tertiary volcanic rocks. 



The contributing basin is underlain mainly by Judith River 

 Formation, with secondary but substantial areas of Bearpaw shale 

 and Tertiary-aged volcanic flows, and minor areas of Colorado Shale 

 and Eagle Formation. 



No direct information regarding hydrology is provided. 



Schrammeck Lake 



Bibliographic source: Montana Bureau of Mines and Geology 

 Publications Index 



Author : Fox , R . D . 



Title: Geology and ground-water resources of the Cascade-Ulm area, 

 Montana 



Series: Montana Bureau of Mines and Geology Bulletin 52 



Date: 1966 



Geologic mapping at 1:62,500 scale shows Schrammeck Lake and most 

 or all of it's contributing basin underlain by the Bootlegger 

 (upper) member of the Blackleaf Formation (of the Colorado Group) 

 of early Cretaceous age. Within the map area, regional dips are 

 to the west and the Bootlegger is unconformably overlain by the 

 Marias River Formation of late Cretaceous age. The map shows no 

 mapped structures in the Schrammeck Lake area other than bedding 

 orientation. 



The Bootlegger Member is described as "interbedded black shale, 

 greenish bentonite, brownish sandstone and chert pebble 

 conglomerate". Bentonitic outcrops are described as exhibiting salt 

 efflorescences. Chert pebble conglomerates are believed to have 

 been derived from erosion of the Permian-aged Phosphoria Formation. 



59 



