Sierra Fresnal
This is one of the few "sources" in northwest Mexico where marekanites are associated with coalesced rhyolite domes. All the dome structures are highly eroded and none exhibited in-situ obsidian zones with marekanites. Indeed, it appears that much of the Sierra Fresnal is composed of rhyolite in a series of northwest-southeast trending coalesced domes. The domes are highly eroded and have produced a regolith of silicic ash, rhyolite lava and obsidian that is eroding east toward Lago Fresnal and Lago Guzman (see map below). Indeed, samples collected from the Arroyo Casas Grandes alluvium 70 km north of Sierra Fresnal exhibit the same elemental concentrations (see Appendix). The Sierra Fresnal collection area is between UTMs 13R 0254349/3446168 and 0249566/3446086 on the INEGI Guzman 1:50,000 H13A42 sheet. The density of nodules near the domes on the east side of the Sierra is up to 50 per 100m2 and down to 1 per 50m2 one km east toward Lago Fresnal. Bipolar cores and flakes occur very rarely.
Sierra Fresnal nodules have been located in alluvium west of the Sierra Fresnal as well at Arroyo Seco south of Nuevo Casas Grandes and at Lago Fredrico (see Chihuahua map below). At Lago Fredrico nodules collected by Alan Phelps just west of Sierra Fresnal exhibit different elemental concentrations, probably derived from a different magma source.
Satellite shaded relief map of a portion of the basin and range region of northeastern Chihuahua showing the location and relationship between the Sierra Fresnal and Lago Fredrico secondary deposits of obsidian (from the INEGI Ciudad Juarez 1:250,000 H 13-1 sheet. Width of grids 1000 meters, sheet oriented north-south.
Sierra Fresnal
N
Minimum
Maximum
Mean
Std. Deviation
Std. Error
Ti
14
1138.00
1219.70
1182.1250
18.1621
36.3242
Mn
14
275.70
314.30
300.7750
8.5965
17.1929
Fe
14
10768.60
11423.00
11169.3250
142.4469
284.8938
Th
14
39.60
61.20
48.2750
4.5803
9.1606
Rb
14
296.90
324.30
316.0000
6.4163
12.8325
Sr
14
34.70
39.90
37.6000
1.1655
2.3310
Y
14
62.30
67.30
64.8000
1.2576
2.5153
Zr
14
164.00
175.40
170.3750
2.3666
4.7331
Nb
14
32.80
37.80
34.3750
1.1821
2.3641
Back to SW Obsidian Sources page
Revised: 19 July 2006
