• There is a general consensus that this was an open marine, shallow shelf or platform setting, possibly not far from the littoral zone. Also, there may have been some form of freshwater influence, judging by the presence of sticks, wood, and other terrigenous material (Vega et al, 2001a; Ferrusquía-Villafranca et al, 2000; Perrilliat et al 2003; Aguilar 1993).

• The San Juan Formation consists of pale yellowish orange 10 YR 8/6, granule to fine gravel, very well indurated, biomicrite/biosparite-matrixed, highly fossiliferous (abundant megaforaminifers), quartz-phyllarenitic conglomerate, set in thick to very thick strata; intercalated with fine grained, quartz-phyllarenitic sandstone of the same color with a biomicrite matrix, set in medium thick strata; by phyllarenitic siltstone of light olive gray 5 Y 6/1 color (weathering to dark yellowish orange 10 YR 6/6) and shaly character, set in thin to laminar strata, that frequently develop cone-in-cone structures. There are also beds of light olive brown 5 Y 5/6 (weathering to dusky yellow 5 Y 6/4), phyllarenitic siltstone of grayish orange pink 5YR712 color (weathering to dark yellowish orange 10 YR 6/6), with plenty of biomicritic matrix, set in thick strata; and highly fossiliferous biomicrite, and biomicrosparite to mesosparite bearing abundant megaforaminifera, set in medium to thick strata; and by light olive gray 5 Y 5/6 (weathering to yellowish brown 10 Yr 5/4), shaly, friable, clayey, phyllarenitic siltstone, set in thick strata. Biostromes dominated by pelecypods, Dasycladaceae and corals, are sparsely distributed throughout the sequence, and may be locally abundant. The formation becomes less limy towards the top. (Ferrusquía-Villafranca et al, 2000).