Page 190 - Carbonate Facies in Geologic History

Algal Plate Mounds at Shelf Margins 177

PARADOX BIOHERMS
SAN JUAN CANYON
SAN JUAN COUNTY
UTAH

MIL!

d "'us FROM .AJNCTION WITH COl.OAAOO AlVER
::=::::: REU TRENDS
_ SlOE V¥£W OF IIIOH£ANS(SUGHTLT EXAGGERATED)
REEFS ARE IN PI>.RADOX FOR"'ATION

, ., ................
"d L.O.' .........
- THE GOOSENEO<S

Fig.VI-4. Trends of early mounds in San Juan Canyon equivalent to Paradox basin evapo-
rites. These trends consist of individual bioherms. The trends are about parallel to the strike
offacies around the Paradox basin. From Wengerd (1963, Fig. 6)

also on the southwestern flank of the Paradox basin (Aneth, Desert Creek, Ismay
fields) where mounds are mainly grainstone accumulations of platy algae (Fig. VI-
2). Algal plate micrite mounds show relief of only 30 m or so but locally slopes of
as much as 25° exist on edges of individual bioherms. Regional slopes into the
basins may be much less, only 1 or 2 degrees at most. Beds on such gentle inclines
are traceable for several km in both the Sacramento and Big Hatchet mountains
of New Mexico. The shelf margin buildups may develop along strips only a few
km broad and on shelves bordering channel-ways into the basins, e.g., Paradox
and Oro Grande. The basins downslope of these shelves were filled reciprocally
with fine clastics and evaporites at times when sea level had dropped exposing the
shelf margin buildups to vadose diagenesis. Basinward sides of these micritic
mounds along the shelves are somewhat steeper. Generally the mounds grew as
chains, in bread-loaf shapes with long axes parallel to the depositional strike,
although exceptions occur. The Ismay field in the Paradox basin shows a trend of
buildups parallel to the shelf edge but with thin, lens-shaped individual mounds
(12 m thick and up to 300 m long) oriented perpendicular to the basin margin, as