New Mexico Geological Society Annual Spring Meeting — Abstracts

More than 1000 m of marine, deltaic, and volcanic strata underlie continental strata of the Lower Cretaceous Hell-to-Finish Formation in the central part of the Little Hatchet Mountains in southwest New Mexico. They are present in extensively faulted exposures on the north, east, and south flanks of Hachita Peak (secs 22-26, T.28 S., R. 16 W.) and southeast of Broken Jug Pass (secs 35 and 36, T. 28 S., R. 16 W., and secs 1 and 2, T. 29 S., R. 16 W.). The strata rest unconformably on Mississippian and younger Paleozoic rocks south of the Copper Dick fault. We correlate these strata with recently discovered Upper Jurassic marine and volcanic strata of the Chiricahua Mountains of southeastern Arizona on stratigraphic position beneath Cretaceous rocks of the Bisbee Group and general lithic similarity of the succession.

Harrigan (1995) divided the section on Hachita Peak into 6 members: the following stratigraphy combines his members 2 and 3, which are difficult to differentiate due to facies change in the area of exposure. In ascending order, the section includes the following members: 1) sandy dolostone member (minimum 140 m); 2) lower conglomerate member (93 m); 3) fine-grained member (474 m); 4) upper conglomerate member (163 m); 5) basalt member (149 m). Member thickness are preliminary because unfaulted sections are difficult to find and may not exist. The lower member consists of thin-bedded tan dolostone and sandy dolostone with fine to medium grains of monocrystulline quartz and detrital chert. Wave Iipples are common and hummocky cross stratification is locally present. Chert fragments are locally abundant, paIticularly above the basal contact with cherty encIinite interpreted as Escabrosa Limestone. The dolostone member represents shoreface deposits. The lower conglomerate member consists of meter-scale beds of matrix-and clast-supported pebble and cobble conglomerate interbedded with coarse-grained carbonate litharenite beds 10-30 cm thick. The member is generally sparsely pebbly in the lower 55 m (unit 2 of Harrigan, 1995). Clasts were deIived from Escabrosa Limestone through Epitaph Dolomite. The member was deposited by turbidity cun'ents, subaqueous debris flows and slumps. The fine-grained member consists of laminated to structureless, fine-grained carbonate litharenite and siltstone with abundant soft-sediment deformation and subordinate matdx-supported conglomerate. It contains uncommon burrows and bivalve shells. The upper conglomerate member is clast-supported conglomerate in beds 2-9 m thick interbedded with mottled carbonate litharenite and siltstone with thick-shelled bivalves and uncommon corals. Limestone, dolostone, chert, and quartzite clasts were derived dominantly from lower Paleozoic formations of the region. The fine-grained and upper conglomerate members were deposited in prodelta and shallow to subaerial fan-delta settings, respectively. The basalt member consists of interbedded vesicular basalt, olivegray and reddish-brown siltstone. and subordinate pebble conglomerate, locally with basalt clasts. These strata originated as subaerial lava flows and deposits of bed-load streams.

The strata described here were originally named Broken Jug Limestone by Lasky (1947). Zeller (1970) reassigned the lower member to his unnamed Cretaceous(?) beds, members 2-4 above to the Hell-to-Finish Formation, and considered the basalt member as Cretaceous-Teniary intrusive diodte. Lucas and Anderson (1996) interpreted the dolostone member as Epitaph Dolomite. We recommend that the name Broken Jug Formation be reinstated for sedimentury and volcanic strata between the Paleozoic and the Hell-to-Finish Formation. The Broken Jug FOlmation represents early deposits of the Borderland rift that extended southeast to the Gulf of Mexico.