Rhaetian (Uppermost Triassic) Radiolarians from the Sandstone Member, San Hipolito Formation, Baja California Sur

Patricia A. Whalen and Elizabeth S. Carter

Radiolarian faunas recovered from the very base of the sandstone member, San Hipolito Formation, Baja California Sur are assigned to the Rhaetian (uppermost Triassic) based on comparison with well-dated Rhaetian radiolarian faunas from Queen Charlotte Islands, British Columbia.

The San Hipolito Formation (Vizcaino Terrane) on the Vizcaino Peninsula, consists of 2400 m of marine sedimentary rocks. The oldest member, a Halobia-bearing chert contains radiolarians assigned to the upper Carnian?/lower to upper Norian (Upper Triassic), lies on pillow basalts and is successively overlain by limestone, breccia and sandstone members. The occurrence of Monotis cf. M. subcircularis (Gabb) in the limestone member indicates an upper Norian age.

Rhaetian radiolarians are found in thin limestone beds at the base of the 1840 m thick volcaniclastic sandstone, about 700 to 900 m below Lower Jurassic (upper Pliensbachian) radiolarians (Whalen and Pessagno, 1984). The Rhaetian age of the lowermost faunas is based on comparison with Rhaetian radiolarian faunas from the Sandilands Formation, Queen Charlotte Islands, British Columbia described by Carter (1993). The age of the Sandilands faunas is determined by co-occurring ammonites and conodonts which permit the construction of Carter's detailed radiolarian zonation for the Rhaetian.

Recognizing the first Rhaetian aged radiolarian faunas and rocks in the San Hipolito Formation better constrains the timing of geologic events for this part of Baja California Sur.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California